2012-01-1134

7/30/2019 2012-01-1134

1/14

INTRODUCTION

In recent years a number of combustion concepts have

been researched in order to reduce engine-out emissions and

increase efficiency. These methods concentrate on low

temperature premixed combustion to alleviate the engine-out

emissions of both NOx and soot while simultaneously

increasing engine efficiency. One such concept,

homogeneous charge compression ignition (HCCI) has

demonstrated significant potential but is faced with a number

of implementation barriers. In order to achieve HCC

combustion in a 4 stroke engine there must be some method

of raising the intake charge temperature to achieve auto-

ignition at or near the TDC of the compression stroke. This

can be achieved with the use of variable valve timing to

produce negative valve overlap (NVO). NVO is achieved by

2012-01-1134Published 04/16/2012

Copyright 2012 SAE Internationadoi:10.4271/2012-01-113

saeeng.saejournals.org

HCCI Load Expansion Opportunities Using a Fully VariableHVA Research Engine to Guide Development of a Production

Intent Cam-Based VVA Engine: The Low Load Limit

Adam Weall, James P. Szybist and K. Dean EdwardsOak Ridge National Laboratory

Matthew Foster, Keith Confer and Wayne MooreDelphi Automotive Systems

ABSTRACT

While the potential emissions and efficiency benefits of HCCI combustion are well known, realizing the potentials on a

production intent engine presents numerous challenges. In this study we focus on identifying challenges and opportunities

associated with a production intent cam-based variable valve actuation (VVA) system on a multi-cylinder engine in

comparison to a fully flexible, naturally aspirated, hydraulic valve actuation (HVA) system on a single-cylinder engine,with both platforms sharing the same GDI fueling system and engine geometry. The multi-cylinder production intent VVA

system uses a 2-step cam technology with wide authority cam phasing, allowing adjustments to be made to the negative

valve overlap (NVO) duration but not the valve opening durations. On the single cylinder HVA engine, the valve opening

duration and lift are variable in addition to the NVO duration.

The content of this paper is limited to the low-medium operating load region at 2000rpm. Using different injection

strategies, including the NVO pilot injection approach, the single-cylinder engine is operated over a load range from

160-390 kPa net IMEP at 2000 rpm. Changes to valve opening duration on the single-cylinder HVA engine illustrate

opportunities for load expansion and efficiency improvement at certain conditions. For instance, the low load limit can be

extended on the HVA engine by reducing breathing and operating closer to a stoichiometric air fuel ratio (AFR) by using

valve deactivation.

The naturally aspirated engine used here without external EGR confirmed that as operating load increases the

emissions of NOx

increases due to combustion temperature. NOx emissions are found to be one limitation to the maximum

load limitation, the other being high pressure rise rate. It is found that the configuration of the production intent cam-based

system represents a good compromise between valve lift and duration in the low to medium load region. Changing the

extent of charge motion and breathing via valve deactivation prove beneficial at moderating the pressure rise rate and

combustion stability and extending the low load limit at 2000rpm on the HVA engine. It also confirms that strategies using

a pilot fuel injection are beneficial at low operating loads but that as operating load is increased, the benefits of multiple

injection diminish to the point where a single injection offers the best performance.

CITATION: Weall, A., Szybist, J., Edwards, K., Foster, M. et al., "HCCI Load Expansion Opportunities Using a Fully

Variable HVA Research Engine to Guide Development of a Production Intent Cam-Based VVA Engine: The Low Load

Limit," SAE Int. J. Engines 5(3):2012, doi:10.4271/2012-01-1134.

____________________________________
http://saeeng.saejournals.org/http://dx.doi.org/10.4271/2012-01-1134

7/30/2019 2012-01-1134

2/14

closing the exhaust valves earlier than in conventional

engines, thereby trapping a portion of the exhaust gases in-

cylinder and recompressing them for the remainder of the

exhaust stroke. The intake valve is then opened later than in a

conventional engine so that the re-compressed exhaust can be

expanded.

HCCI operation is limited to part-load conditions due to

high rates of in-cylinder pressure rise rate, making itapplicable to only a small portion of the engine map. The

higher efficiency at part-load conditions is challenged by the

growth of hybrid-electric powertrains in production vehicles,

which minimize the use of the engine at the lowest engine

loads and operate the engine at more efficient higher-load

conditions whenever possible. Thus, if HCCI is to remain a

relevant means of increasing efficiency, there is a need for a

greater emphasis on expanding the operating regime both to

lower loads and to higher loads. More recently it has been

shown that operating loads approaching idle speed conditions

can be achieved using recompression effects combined with

late fuel injection and spark-assistance [1]. The achievement

of high load HCCI operation continues to be investigated

using methods such as intake charge boosting, stratification

and spark-assisted operation [3,4,5,6,7,8].

The requirement for a complex combustion control

system combined with the need for an advanced valvetrain

possessing a large degree of freedom has been confirmed in

many HCCI studies. At the same time the sophistication of

cam-based variable valve actuation (VVA) technology has

progressed significantly and it is now feasible for a valve

train with both intake and exhaust valve timing phasing as

well as 2-step lift to be used on a production-intent engine

platform [9].

The work presented in this paper explores the mechanismof multiple injection in a HCCI engine, then examines the

effect of changing the valve train parameters in terms of

valve lift, valve timing and NVO duration. Finally the effect

of intake valve deactivation is investigated on HCCI

combustion at the low load limit. The experimental

investigation presented here is performed using a naturally

aspirated single-cylinder engine with fully variable hydraulic

valve actuation system supplied by Sturman Industries. The

valve train approach is based around the design-space

envisaged in a production-intent VVA multi-cylinder engine.

The same compression ratio, fuel injection equipment (FIE)

and fuel (E10) being used in an ongoing study on the multi-

cylinder platform is used on the single-cylinder platformpresented in this paper.

EXPERIMENTAL SETUP

SINGLE CYLINDER ENGINEA highly modified GM 2.0L Ecotec engine with direct

fuel injection is used for the study. The engine geometry is

listed in table 1. Three of the cylinders of the production

engine are disabled to allow single-cylinder operation, a

custom piston is installed to increase compression ratio to

11.85, and the engine is operated naturally aspirated without

external EGR.

The single cylinder engine is equipped with a hydraulic

valve actuation (HVA) system, allowing independent contro

of each of the two intake and two exhaust valves, including

opening and closing angle, opening duration and valve lift

including the option to disable a valve. While this makes it a

very versatile research tool for this type of research, it islimited by the fact that the valve opening profile differs from

a cam-based valve train. The rapid valve opening, dwell a

maximum lift, and rapid valve closing give the valve even

the appearance of a square wave rather than a conventional

cam profile. This is discussed further in the appendix.

Machining modifications have been made to the cylinder

head to accommodate the small research module HVA

system from Sturman Industries. Modifications include using

custom intake and exhaust valves with longer valve stems

While the valve material is different than the production

valves, the combustion chamber geometry is unchanged

Other changes to the engine include a custom exhaust system

and a Kistler sparkplug with an integrated piezoelectric

pressure transducer. In the production configuration from the

original equipment manufacturer (OEM), the high pressure

fuel pump for the gasoline direct injection (GDI) fueling

system is driven by the intake cam shaft. However, because

the cam shafts are removed, the fuel is supplied by an

externally powered fuel pump. A picture of the engine is

shown in figure 1.

Figure 1. Single cylinder engine.

Table 1. Engine geometry.

Drivven Combustion Analysis Toolkit (DCAT) performs

the crank-angle resolved data acquisition and combustion

analysis. These measurements include cylinder pressure

Weall et al / SAE Int. J. Engines / Volume 5, Issue 3(August 2012)

7/30/2019 2012-01-1134

3/14

valve lift feedback from each of the four valves and the

command signal sent to the fuel injector. Crank-angle

resolved data is recorded at 0.2degCA intervals over 300

consecutive cycles, and all references to indicated mean

effective pressure (IMEP) refer to net IMEP.

Engine emissions are measured using a standard

emissions bench. NOX is measured using a

chemiluminescence analyzer, CO and CO2 are measured

using infrared analyzers, oxygen is measured using a

paramagnetic analyzer and unburned hydrocarbon emissions

(HC) are measured with a flame ionization detector. Smoke

measurements are performed using a filter smoke number

(FSN) instrument. Exhaust air-to-fuel ratio (AFR) is

measured using both gaseous emissions and a wideband

exhaust lambda sensor.

Air mass flow is measured using a laminar flow element

device and fuel flow is measured using a coriolis-effect based

flow meter. Fuel rail pressure is regulated to a constant 100

bar. Engine coolant is maintained at 90C. The air supplied to

the engine intake manifold is externally conditioned to 55%relative humidity and 25C using an air supply conditioning

unit. The intake air temperature in this paper is measured

inside the intake manifold before the intake port. The exhaust

temperature is measured in the exhaust manifold directly after

the exhaust port.

FUEL PROPERTIESThe single cylinder engine is operated using E10 with the

specifications given in Table 2. The ISFC values presented in

this paper are calculated using the net IMEP and the fuel

mass flow rates measured by the coriolis fuel meter and are

not normalized to the energy value of an E0 gasoline.

Table 2. Test fuel.

ENGINE OPERATING STRATEGY

SINGLE CYLINDER ENGINEA baseline HCCI operating point is defined at 300 kPa

IMEP and 2000rpm. At this operating load we first

investigate effects of fuel injection parameter variation and

valve characteristics before increasing or decreasing

operating load to the limits of operation.

The limits of operation for the purposes of the single

cylinder experiments are an upper limit of the coefficient o

variance (COV) of IMEP of 5% and a maximum ringing

intensity of 5 MW/m2. The RI relation proposed in [3] is

applied in this paper and is reproduced below.

The limit of RI corresponds to a maximum pressure rise

rate (MPRR) of 300 to 400 kPa/degCA given the range of

peak pressure (PP) encountered at an engine speed of 2000

rpm. This is a conservative limit that would take into accoun

the production intent limitations placed on a light duty

engine. When multiple injection is used, the mass fraction

between the pilot and the main injection event reported in this

paper is measured by injecting fuel at the same operating

condition without pilot injection while the engine is

motoring. The fuel injection mass is measured by the coriolis

fuel meter and the mass fraction is then determined. The large

separation of the two injection events reduces the likelihoodof injector interference and pressure wave effects to a

minimum.

RESULTS

SINGLE CYLINDERThe baseline HCCI operating point uses 4 valve operation

and 3mm lift. The HVA valve lifts are shown in figure 2

Here the characteristic rise and fall of the HVA system is

evident which possesses some inherent asymmetry as

indicated. The choice of HVA valve duration was based upon

the profile of the production intent HCCI cam profile shown

in figure 2.

Figure 2. HCCI operating entry point : HVA valve lift

profile and VVA cam profile.

The negative valve overlap (NVO) duration of the

baseline HCCI operating point is 180 degCA with intake and

exhaust valve event durations of 104 degCA assuming

0.2mm lift. For a HCCI engine operating with NVO, the


7/30/2019 2012-01-1134

4/14

effect of direct fuel injection parameters is highly significant

in contrast to a PFI engine where there is effectively no way

to either adjust mixture stratification or to take advantage of

fuel interactions during the NVO period. Numerous studies

into the effects of direct injection in HCCI engines exist with

most proposing the use of a pilot fuel injection before or

during the NVO period followed by a main injection event

after the NVO period has passed. A comparison of single andmulti-injection in the single cylinder engine follows.

Single Injection Event Operation (4Valve - 3mm

Lift)

The effect of first using a single injection event is

examined using the valve timing shown above while

adjusting fueling to maintain 300 kPa IMEP load. The fuel

injection parameter varied is DI timing and its variation can

be seen in terms of secondary variables in figures 3, 4 and 5.

Spark assist is applied with a 20degATDC spark event for

stability.

Figure 3. Single injection SOI (COV of IMEP and RI).

Figure 4. Single injection SOI (ISFC and CA50).

Figure 5. Single injection SOI (NOxand Lambda).

The effect of injection timing is significant. The range of

stability (COV) and noise (RI) limits can be spanned with a

DI timing change from 335 to 365 degATDC SOIparameter variation. At the same time the combustion

phasing responds within 5degCA for the same SOI variation

and ISFC is minimum at an SOI of 340degCA at

combustion phasing of 8degCA ATDC. This does indicate

some control scope over combustion phasing and it also

shows that operation with less than 2% COV, RI of less than

3 MW/m2 and 235 g/kWh can be achieved within this

parameter space, while keeping the valve timing constant.

In order to examine the effect of valve duration the

previous configuration is re-measured with a valve duration

increased from the baseline 104degCA to 112degCA. The

valve events are centered around their middle anchor so this

means that the duration increase advanced EVO and IVO

and retarded EVC and IVC values. A single direct injection

SOI of 320 degATDC is chosen. The effect of spark at this

lean operating point is to advance the combustion phasing by

1degCA, leading to a minimal decrease in ISFC of around

1-2 g/kWh compared to the condition where the spark is not

used. However, the predominant effect is the change in valve

timing. The results are plotted in figures 3,4,5 as single

points, where it can be seen that for the optimal combustion

phasing of 8-9degCA ATDC, ISFC was reduced to 226

g/kWh while COV was marginally over 3%. NOx

concentration was well below 10ppm.

To summarize, increasing valve duration from 104degCAto 112degCA at this operating point reduces fue

consumption while operating with acceptable combustion

stability. This indicates that optimization of valve cam

profiles in a production VVA engine can lead to reductions in

ISFC. Nevertheless it must be considered that there exist

physical limitations to the design of a conventional cam

profile in terms of durability and that the effects of engine

breathing seen on a HVA valvetrain may well differ from a

cam-based VVA system. For these reasons, overall trends


7/30/2019 2012-01-1134

5/14

rather than specific optimizations are emphasized in this

paper.

Multiple Injection Event Operation (4Valve -

3mm Lift)

The previous section has shown that it is possible to

operate with a single injection event occurring after the NVO

period and before the intake valve opens. The use of amultiple injection strategy with a pilot event during NVO and

a main event after NVO has been investigated in several

publications where both exothermic reactions and fuel

reforming leading to a greater propensity for auto-ignition to

occur have been identified, [10,11,12,13,14,15]. These

mechanisms would also be present in the case of a single

injection with SOI during the NVO period which explains the

change in combustion phasing seen in figures 3,4,5.

Comparable effects are seen in spark-assisted HCCI when a

single injection event is advanced toward the NVO period,

[3].

Pilot Injection Quantity and Pilot Injection SOIThe results shown here investigate the effect of pilot

injection using the same valve timing as the previous section

with 104degCA duration shown in figure 2. In this section the

main injection SOI is 240degCA ATDC (occurring during

the intake valve event) and the pilot injection quantity and

SOI is varied. In these tests a pilot injection event is

necessary when the main injection event timing is retarded to

this magnitude. This can also be seen by examining figure 3

where single injection retard leads to increased levels of COV

of IMEP. Therefore at this level of fueling the interaction

(exothermic and/or fuel reforming) between fuel injection (be

it single or multiple injection) and NVO is necessary for

stable combustion to occur.

The multiple injection strategy used holds the mass

fraction of pilot to main injection at 5% and 15% and then

varies the pilot SOI. As with all tests the overall fueling is

adjusted to maintain a constant load (300 kPa IMEP in this

section), involving minor trimming of fuel injection

durations. The results are shown in figures 6,7,8. Fuel

consumption is approximately constant at 232 g/kWh which

shows a reduction of 4g/kWh compared with the single

injection used in the previous section and NOx concentrations

below 15ppm. Combustion phasing is affected considerably

by pilot SOI in a similar manner to that seen with a single

injection in the previous section.The combustion stability and RI tradeoff seen in figure 6

is comparable again with the single injection results. This

confirms that the SOI of the pilot injection does have

significant effect over the combustion event. The pilot

quantity has a complimentary effect however where it may be

possible to use either pilot SOI variation OR pilot quantity

variation as a control parameter. This would allow for

example the use of a fixed pilot quantity which has certain

advantages in terms of FIE hardware linearity and response

that will be discussed later in this paper. In terms of pilot

quantity it can also be observed that its effect reduces as the

SOI approaches 360 deg ATDC. This would suggest that the

exothermic and/or fuel reforming mechanism is reduced

when the fuel has less residence time in the NVO high

pressure and temperature phase of the NVO period.

By examining figures 6 and 7 it can be seen that the

higher pilot quantity advances combustion phasing

significantly and leads to an increase in the ringing intensityhowever operation at a reduced RI of much less than 5

MW/m2 is possible with a range of pilot quantity and pilot

SOI configurations.

Figure 6. Pilot injection SOI (COV of IMEP and RI).

Figure 7. Pilot injection SOI (ISFC and CA50).


7/30/2019 2012-01-1134

6/14

Figure 8. Pilot injection SOI (NOxand Lambda).

Main Injection SOI

While the effect of pilot injection strategy during theNVO of an HCCI engine is widely reported there is less

investigation into the effect of the main injection SOI. It is

generally thought that the SOI of the main injection should be

early enough to allow adequate mixing but at the same time

shouldn't interact with the NVO period in the same way that

the pilot injection does. In this study an SOI of

240degATDC was initially used which means the fuel is

injected during the intake valve event. The possible effect of

the main SOI is examined by holding the pilot injection

constant at 393degCA ATDC with a pilot mass fraction of

5%. A number of measurements are then taken using main

SOI of 280 and 240deg ATDC. The injection strategy is

shown in figure 9 where the intake valve interaction with the

main injection can be seen. Using a spark event at 40deg

ATDC it is possible to achieve the results shown in table 3.

Figure 9. Multiple fuel injection strategy.

Table 3. Effect of main injection SOI.

Both conditions are comparable in terms of combustion

phasing and ISFC but differ considerably in terms of COV o

IMEP and RI. The main injection event during the intake

valve opening duration leads to cyclic variation of peak

pressure rise rate and results in a greater variation in IMEP

This shows that in a direct injection HCCI engine

consideration must be given to the timing of the main

injection event. The effect shown here was particularly

sensitive to the amount of fuel injected. At fueling rates lower

than used at this operating point this effect of main SOI is no

seen. For reference, the filter smoke numbers do not vary

during these tests and remain at the limit of the detection

ability of the smoke meter, as is the case in all other HCCI

measurements presented in this paper.

To summarize, the results of this sub-section investigating

the effects of fuel injection parameters show the potential for

combustion phasing adjustment. The optimization of valve

timing using a 3mm lift profile and multiple injection

strategies shows stable combustion exhibiting moderate

combustion noise with ISFC at or below 230 g/kWh at a load

of 300 kPa IMEP.

Low Load Operating Limit (3mm Lift Including

Intake Port Deactivation)

It has been shown that lean HCCI operation is possible at

300 kPa IMEP using either a single fuel injection event early

in the compression stroke or multiple injection events

involving a pilot injection during the NVO period

Combustion stability at this operating load does not presen

significant limitations, emissions of NOx remained

acceptably low and overall efficiency was high.

In contrast, the level of combustion instability increases

for a reduction of operating load to below 300 kPa IMEP at

2000 rpm. In this operating region the use of the pilot-NVO

approach is found to be beneficial when compared with

single injection operation. The interaction between this andthe effects of valve timing and duration is discussed in this

sub-section (valve lift is maintained at 3mm).


7/30/2019 2012-01-1134

7/14

At low operating loads, in order to maintain combustion

stability at or below a limit of 5% COV of IMEP the

application of both single injection and multiple injection is

attempted. The multiple injection strategy with a pilot event

during NVO is found to achieve the best stability to ISFC

tradeoff. However the sensitivity of combustion phasing to

pilot quantity and pilot timing is high. The sensitivity to pilot

SOI for a fixed pilot mass is found to be higher than for afixed pilot SOI with variation of pilot mass. i.e. pilot timing

sweeps may be a more effective control parameter, which

correlates with previous findings [12,13]. Nevertheless, the

amount of fuel injected during the NVO remained important

and it is found necessary to inject a greater mass fraction of

fuel in the pilot event when compared with the higher 300

kPa IMEP operating point.

Stable HCCI operation (defined as COV IMEP

7/30/2019 2012-01-1134

8/14

figures 10,11,12 show an increased valve duration combined

with an increased NVO fuel injection strategy is beneficial in

terms of fuel consumption and stability.

Also shown on figure 13 is the HVA 3mm lift profile that

was used in the previous section and shown in figure 2. It can

be envisaged that a single cam profile similar to that shown

would be ideal for spanning the low load limit through the

low-mid range of 300 kPa IMEP with the use of valvephasing and fuel injection parameter control. It also

emphasizes the control possibilities offered by fuel injection

parameters and their ability to reduce the necessary degrees

of freedom present in the valve train.

Figure 13. HVA and VVA valve profiles (valve duration

variation).

The use of valve deactivation at low operating loads

shows benefit in terms of combustion stability. When

operating with the largest valve duration and 10% pilotinjection mass there is evidence that intake valve deactivation

improved the stability-ISFC tradeoff. The mixture lambda

value decreased with intake valve deactivation from 1.55 to

1.35 at the leanest operating points in figure 12. Emissions of

NOx show no significant change as a result of this change in

global AFR at this operating point.

Further Reduction of Operating Load

A further reduction in operating load to 160 kPa IMEP is

possible at 2000rpm with the use intake valve deactivation.

This was made possible by the increased stability exhibited

when operating nearer to stoichiometric AFR. The valve

profiles used at the 200 kPa and 160 kPa IMEP operatingpoints can be seen in figure 14 with the inclusion of the 3mm

HCCI cam profile. The production-intent cam phasing system

is therefore indicated to be able to operate at this low load

limit especially when we consider that at low valve lifts the

HVA profile matches better the cam profile and would be

expected here to represent the cam profile shown very well.

Figure 14. HVA and VVA valve profiles (low load limit).

Figure 15. Low load limit cylinder pressure.

The experimental in-cylinder pressure along with the

results from the GT-Power model is shown in figure 15 for

the 160 kPa operating point. The injection strategy and spark

timing is seen in table 4. Pilot fraction is 50% of the injected

mass and the pilot SOI is 15 degrees before the TDC of gas

exchange. This is the most effective pilot SOI at this

operating point enabling a significant increase in combustion

stability to be achieved. COV of IMEP is 5.4% and the RGF

is calculated as 35% from the GT-Power model.

Table 4. Low load operating limit parameters.


7/30/2019 2012-01-1134

9/14

Thus, a 3mm valve lift profile with the HVA system

allows stable operation at 300 kPa IMEP as well as stable

operation at operating loads as low as 160 kPa IMEP with

intake valve deactivation. The next subsection presents an

overview of the maximum load under lean conditions.

Lean HCCI Load Increase Limit (Naturally

Aspirated Operation Without External EGR)At operating loads above 300 kPa IMEP at 2000rpm it is

found that a pilot fuel injection into the NVO is detrimental

to engine performance because of overly advanced

combustion phasing, and the associated high RI and high

NOx emissions. Instead, these points are operated with a

single fuel injection. Intake and exhaust valve phasing is used

with a HVA valve lift profile comparable to the 3mm VVA

cam based profile shown in figure 2. At operating loads

above 200 kPa IMEP valve deactivation is not used and the

engine operated in 4 valve mode.

Figure 16 shows the ISFC as load is increased for

maximum torque spark timing on the single cylinder engine.

Also shown is the ISFC measured for lean HCCI with the low

load limit operating with intake valve deactivation. The COV

of IMEP increases to just over the 5% COV of IMEP limit

and the tradeoff for combustion stability and combustion

noise (in terms of ringing intensity) is shown in figure 17

with the highest load exhibiting 5-6 MW/m2. SI combustion

is not normally evaluated in terms of ringing intensity and is

not included on figure 17 (for reference the value of RI was

less than 1 MW/m2 for the SI combustion points).

The NOx emissions and lambda are shown in figure 18.

Here we see that the NOx emissions increase with load from

under 5ppm at the low load limit up to nearly 60ppm as the

limit of ringing intensity is reached. In further experiments ithas been confirmed that any further increase in operating load

led to increases in combustion noise and a rapid increase in

the emissions of NOx to levels above 150ppm.

Figure 16. Load variation for SI and lean HCCI (ISFC).

Figure 17. Load variation for SI and lean HCCI (COV

of IMEP and RI).

Figure 18. Load variation for SI and lean HCCI (NOx

and lambda).

The results shown in this section correlate well with those

of previous work and show that both NOx and combustion

noise become limitations when a lean HCCI engine platform

is operated at higher loads. Numerous other methods o

increasing the operating range of HCCI and related advanced

combustion strategies, and include include intake air charge

boosting [6], fuel injection stratification [8,8] and

stoichiometric spark assisted operation [3,4,5]. Intake charge

boosting combined with external EGR are the planned nex

stages on this engine platform, however the scope of this

study is limited to naturally aspirated lean HCCI, and it isinformative to examine the upper load limitations under these

conditions.

DISCUSSION

It can be seen that NVO pilot injection during the NVO

period is not necessary on this engine platform at loads o

300 kPa and higher. It is found that stable HCCI operation is

possible at 300 kPa IMEP without the pilot NVO strategy by

using a moderate advance of main SOI from 280 to 300

degATDC. At higher operating loads the NVO strategy leads


7/30/2019 2012-01-1134

10/14

to advance of combustion phasing and an increase of the

ringing intensity with non-optimal phasing in terms of ISFC.

In contrast at the operating load of 200 kPa it is beneficial to

use the pilot NVO strategy. This correlates well with findings

elsewhere that showed that the requirement for NVO pilot

was limited in the 300-400 kPa IMEP operating region [16].

In terms of the ratio of fuel injected in the pilot it is found at

the lowest load studied of 160 kPa IMEP that a 50% split ofpilot and main fuel injection exhibits combustion stability of

approximately 5% COV of IMEP. A recent study of near idle

conditions in a HCCI engine also confirmed that a 50% pilot

injection was necessary to maintain acceptable combustion

stability with the pilot injection timing occurring earlier than

370deg ATDC [1]. The optimal pilot SOI in the present

study was found to be 375deg ATDC in terms of low load

stability.

Investigation of intake valve deactivation on a VVA GDI

engine [17] confirms that in-cylinder air-fuel distribution is

reduced as a result of the increase in charge motion leading to

a more uniform air-fuel mixture in a direct injection gasoline

engine. This correlates with similar findings found elsewhere

[18]. An increase in combustion stability at low operating

loads was reported in the GDI engine studied in [17] which

incidently is the same base engine design as the single

cylinder engine in this paper operating with comparable main

fuel injection timings. The increase in low load stability in

[17] and [18] is found when operating a stoichiometric GDI

engine not a HCCI engine. However it is interesting to note

that during the present study at the low load limit the

presence of a spark event was necessary to increase

combustion stability and that deactivating an intake valve

reduced the air fuel ratio. This may suggest that the spark

assist was more effective when the air fuel ratio was richer.It can be argued that the effect of valve deactivation on

reducing the in-cylinder AFR could also be achieved by

increasing the NVO duration and/or reducing the valve lift.

Firstly, an increase in NVO duration (and increase in trapped

gas fraction) could increase combustion instability with the

colder exhaust gas exhibited at the low load limit. Secondly, a

reduction in valve lift (and duration) from the 3mm profile

would seem impractical compared to the ability to deactivate

an intake valve, with 4 valve operation used at increased

operating loads. The reasons for an increase in low load

stability found in HCCI operation when deactivating an

intake valve in terms of in-cylinder flow i.e. swirl are beyond

the scope of this study and require further investigation.

LOW FLOW INJECTOR

CHARACTERISTICSThe strategy to reach the low load limit of 160 kPa IMEP

uses 3mm valve lift and intake valve deactivation. It must be

mentioned that the pilot injection strategy used at the low

operating loads was subject to the limitations of a standard

GDI injector which is not designed to accommodate the low

flow rates used during pilot injection. Experimentally, this

posed difficulties in terms of the response between injecto

duration and injected mass. Advances in fuel injector

technology advance rapidly and an improvement in the low

flow response of the injector would have great benefits in for

example control stability.

Current efforts in industry strive to meet requirements tha

are beneficial to multi injection HCCI operationa

requirements. Implementation of multiple injection SIsystems for the purposes of reducing particulates or

improving fuel economy are of increasing interest. The need

for small quantities of fuel to be injected separately from a

main homogeneous injection while supporting a dynamic

range of a naturally aspirated or boosted gasoline engine a

full load is goal that the industry is working to meet and is

one that will be synergistic with the needs of a HCCI mult

injection FIE. The requirements of such a system for HCCI

involve delivering a high quality fuel spray at quantities at or

below 1mg/pulse in some cases. The precision of the fue

delivery at these small quantities is of the utmost importance

as it contributes directly to the consistency of the NVO

exothermic reactions supporting low load HCCI combustion

Variations in quantity or quality of the injection at this poin

lead directly to variations in combustion phasing and IMEP.

Efforts to address the needs for low flow injection

capabilities while maintaining full load dynamic range is a

challenge the industry is working to meet as a system

solution. The complete FIE must be considered in this

endeavor as design aspects of the DI pump, fuel rail, injector

and injector driver can all contribute negatively to the

achievement of a full dynamic range - low flow injector

design if not carefully considered during the design phase o

the FIE. With improved FIE hardware, it should be possible

to further optimize performance and control of HCCIcombustionprocesses

VVA VALVETRAIN

IMPLEMENTATIONThe investigation into valve lift and duration made

possibleby the fully variable HVA system showed that the

3mm cam profiles used in the production-intent multi-

cylinder engine was near optimal in its ability to cover the

low to medium 2000rpm load range in combination with

appropriate fuel injection parameter settings.

Although not presented here, the higher lift and longer

duration offered by a 4mm lift profile was also found to be

able to effectively cover a large part of the operating region

shown using both intake and exhaust valve deactivation while

the lower lift and shorter duration 3mm profile type was used

effectively at the low load limit region using intake valve

deactivation alone. Following further optimization of pilo

NVO strategies, a valvetrain using one lift profile on one

intake port and one exhaust port may well be able to

accommodate HCCI operation at 2000rpm. This

simplification would reduce the required complexity of a

production-intent valvetrain and easily allow for alternative


7/30/2019 2012-01-1134

11/14

cam profiles to be available for mode switching and for

conventional SI operation. Of course the addition of a low lift

cam profile intended for operation at different engine speeds

than 2000rpm would also be feasible. Nevertheless, the

opportunity for valvetrain simplification is indicated due to

the wide control space offered by a complex fuel injection

system capable of two or more injection events.

Since the HCCI operational domain exists at residuallevels not supported by spark ignited engines, it is difficult

for commercial valvetrain systems to accommodate the

breathing requirements of both modes of operation. A four

valve system requires an abrupt change in the breathing of the

engine employing modifications to phasing, changing cam

profiles and de-throttling simultaneously to a high residual,

de-throttled breathing mode. This abrupt change, regardless

of the valvetrain approach used, is difficult to manage in the

transition between combustion modes as the transient

changes in breathing always blend residual mixtures

incompatible to robust combustion for both SI and HCCI

modes. Advanced valvetrain solutions becoming available

may alleviate some of the controls burden placed on throttle

or phasing control while maintaining robust combustion and a

balanced torque output for four valve HCCI/SI transition and

control.

A two valve implementation, where one of the intake and

exhaust valves are deactivated depending on whether the

engine is operating in a low load SI or a HCCI combustion

mode, may allow for the valvetrain to dedicate cam profiles

and phasings per valve for low load to either SI or HCCI

operation. This application may have the benefit of utilizing a

lightly throttled or de-throttled SI cam profile and phasing.

Thi would decrease the controls complexity required for

smooth HCCI/SI transitions. The application of such a systemrequires the exhaust and intake systems also have a higher

load cam profile set that may operate with all four valves

active. This approach is available today through commercial

valve deactivation and multi-step valvetrain systems and is

just one of the potential solutions to the challenges the

valvetrain systems face to transition across the mutually

exclusive domains of the SI and HCCI during low load

operation.

CONCLUSIONS

The first central finding of this study is that the load range

that can be attained for naturally aspirated HCCI operation

does not require the full authority of an HVA valve train, andinstead can be done with the authority provided by a flexible

production-intent cam-based VVA system. While valve

profiles and NVO duration are important parameters, the fuel

injection strategy can be used to control the engine through a

range of engine loads. Further, it is a valve deactivation

strategy rather than a change in valve lift profile that enables

the lowest load point of 160 kPa IMEP, making it more

compatible for production-intent platforms.

The second central finding of this study is a pilot fue

injection event into the NVO is advantageous at the lowes

engine loads, but becomes detrimental at engine loads greater

than 300 kPa IMEP. The increase in temperature and/or

enhanced fuel reactivity that is accomplished through NVO

pilot injection is necessary of operation at the lowest loads

However, at loads above 300 kPa IMEP it leads to overly

advanced combustion, high rates in cylinder pressure riseand an increase in NOx emissions.

Additional findings are as follows

The effect of fuel injection parameters in terms of pilot

timing and quantity was to advance or retard the combustion

phasing and allow some control of combustion phasing.

RI and COV of IMEP are sensitive to the timing of of the

main fuel injection at a load of 300 kPa IMEP. The more

advanced main injection increased stability and reduced

COV. ISFC was not affected.

The combustion phasing was sensitive to the SOI of the

main injection event when operating without pilot injection at

higher operating loads.

The use of a split injection strategy at low loads to increase

combustion stability shows benefits.

There is no evidence for a significant increase in ISFC

when using a single fuel injection at higher operating loads.

Intake valve deactivation allows the low load limit to be

expanded to a high idle operating point

Valvetrain solutions supporting transitions to and from

HCCI and SI, lessening burdens on controls systems, are

becoming increasingly available.

A systems approach must be implemented when developing

the FIE to be capable of low flow, multi injection strategies

for HCCI and maintain full dynamic range.

REFERENCES1. Wermuth, N., Yun, H., and Najt, P., Enhancing Light Load HCC

Combustion in a Direct Injection Gasoline Engine by Fuel ReformingDuring Recompression, SAE Int. J. Engines 2(1):823-836, 2009, doi10.4271/2009-01-0923.

2. Urushihara, T., Yamaguchi, K., Yoshizawa, K., and Itoh, T., A Studyof a Gasoline-fueled Compression Ignition Engine - Expansion of HCCOperation Range Using SI Combustion as a Trigger of CompressionIgnition, SAE Technical Paper 2005-01-0180, 2005, doi10.4271/2005-01-0180.

3. Yun, H., Wermuth, N., and Najt, P., Extending the High LoadOperating Limit of a Naturally-Aspirated Gasoline HCCI CombustionEngine, SAE Int. J. Engines 3(1):681-699, 2010, doi10.4271/2010-01-0847.

4. Szybist, J., Nafziger, E., and Weall, A., Load Expansion oStoichiometric HCCI Using Spark Assist and Hydraulic ValveActuation, SAE Int. J. Engines 3(2):244-258, 2010, doi10.4271/2010-01-2172.

5. Manofsky, L., Vavra, J., Assanis, D., and Babajimopoulos, A.Bridging the Gap between HCCI and SI: Spark-Assisted CompressionIgnition, SAE Technical Paper 2011-01-1179, 2011, doi10.4271/2011-01-1179.

6. Dec, J. and Yang, Y., Boosted HCCI for High Power without EngineKnock and with Ultra-Low NOx Emissions - using ConventionaGasoline, SAE Int. J. Engines 3(1):750-767, 2010, doi10.4271/2010-01-1086.

http://dx.doi.org/10.4271/2010-01-1086http://dx.doi.org/10.4271/2011-01-1179http://www.sae.org/technical/papers/2011-01-1179http://dx.doi.org/10.4271/2010-01-2172http://dx.doi.org/10.4271/2010-01-0847http://dx.doi.org/10.4271/2005-01-0180http://www.sae.org/technical/papers/2005-01-0180http://dx.doi.org/10.4271/2009-01-0923

7/30/2019 2012-01-1134

12/14

7. Dec, J., Yang, Y., and Dronniou, N., Boosted HCCI - ControllingPressure-Rise Rates for Performance Improvements using Partial FuelStratification with Conventional Gasoline, SAE Int. J. Engines 4(1):1169-1189, 2011, doi:10.4271/2011-01-0897.

8. Dahl, D., Andersson, M., Berntsson, A., Denbratt, I. et al., ReducingPressure Fluctuations at High Loads by Means of Charge Stratificationin HCCI Combustion with Negative Valve Overlap, SAE TechnicalPaper 2009-01-1785, 2009, doi:10.4271/2009-01-1785.

9. Hendriksma, N., Kunz, T., and Greene, C., Design and Development ofa 2-Step Rocker Arm, SAE Technical Paper 2007-01-1285, 2007, doi:10.4271/2007-01-1285.

10. Koopmans, L., Ogink, R., and Denbratt, I., Direct Gasoline Injection inthe Negative Valve Overlap of a Homogeneous Charge CompressionIgnition Engine, SAE Technical Paper 2003-01-1854, 2003, doi:10.4271/2003-01-1854.

11. Urushihara, T., Hiraya, K., Kakuhou, A., and Itoh, T., Expansion ofHCCI Operating Region by the Combination of Direct Fuel Injection,

Negative Valve Overlap and Internal Fuel Reformation, SAE TechnicalPaper 2003-01-0749, 2003, doi:10.4271/2003-01-0749.

12. Song, H. and Edwards, C., Optimization of Recompression Reactionfor Low-Load Operation of Residual-Effected HCCI, SAE TechnicalPaper 2008-01-0016, 2008, doi:10.4271/2008-01-0016.

13. Song, H., Padmanabhan, A., Kaahaaina, N. and Edwards, C.,Experimental study of recompression reaction for low-load operation indirect-injection homogeneous charge compression ignition engines withn-heptane and i-octane fuels, International Journal of Engine ResearchAugust 1, 2009 vol. 10 no. 4 215-229, doi:10.1243/14680874JER03309.

14. Aroonsrisopon, T., Nitz, D., Waldman, J., Foster, D. et al., AComputational Analysis of Direct Fuel Injection During the NegativeValve Overlap Period in an Iso-Octane Fueled HCCI Engine, SAETechnical Paper 2007-01-0227, 2007, doi:10.4271/2007-01-0227.

15. Waldman, J., Nitz, D., Aroonsrisopon, T., Foster, D. et al.,Experimental Investigation into the Effects of Direct Fuel InjectionDuring the Negative Valve Overlap Period in an Gasoline Fueled HCCIEngine, SAE Technical Paper 2007-01-0219, 2007, doi:10.4271/2007-01-0219.

16. Persson, H., Pfeiffer, R., Hultqvist, A., Johansson, B. et al., Cylinder-to-Cylinder and Cycle-to-Cycle Variations at HCCI Operation WithTrapped Residuals, SAE Technical Paper 2005-01-0130, 2005, doi:10.4271/2005-01-0130.

17. Moore, W., Foster, M., Lai, M., Xie, X. et al., Charge Motion Benefitsof Valve Deactivation to Reduce Fuel Consumption and Emissions in aGDi, VVA Engine, SAE Technical Paper 2011-01-1221, 2011, doi:10.4271/2011-01-1221.

18. Mittal, M., Hung, D., Zhu, G., and Schock, H., High-Speed Flow andCombustion Visualization to Study the Effects of Charge MotionControl on Fuel Spray Development and Combustion Inside a Direct-Injection Spark-Ignition Engine, SAE Int. J. Engines 4(1):1469-1480,2011, doi:10.4271/2011-01-1213.

CONTACT INFORMATION

James P. Szybist

[email protected]

Matt Foster

[email protected]

ACKNOWLEDGMENTS

Research sponsored by the U. S. Department of Energy,

Office of Energy Efficiency and Renewable Energy, Vehicle

Technologies Program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. It is also

performed under cooperative research and development

agreement (CRADA) no. NFE-10-02739 between UT-

Battelle, LLC and Delphi Automotive Systems LLC.

DEFINITIONS/ABBREVIATIONS

ATDCAfter top dead centre

CA50Crank position at 50% of cumulative apparent heat release

COVCoefficient of variance (net unless otherwise stated)

DIDirect injection

EVOExhaust valve opening

EVCExhaust valve closingEV

Exhaust valveFIE

Fuel injection equipmentHCCI

Homogeneous charge compression ignitionISFC

Indicated specific fuel consumption (not normalized forE10 fuel)

IVOIntake valve opening

IVCIntake valve closing

IVIntake valve

LIVCLate intake valve closing

MPPRMaximum pressure rise rate

NANot available

NVONegative valve overlap

PPPeak pressure

RGFResidual gas fraction

RIRinging intensity

SA-HCCISpark-assisted homogeneous charge compression ignition

SISpark ignition

2V2 valve operation (one intake and one exhaust valvedeactivated)

4V4 valve operation (one intake valve deactivated whennoted)

http://dx.doi.org/10.4271/2007-01-0219http://www.sae.org/technical/papers/2008-01-0016http://dx.doi.org/10.4271/2008-01-0016http://www.sae.org/technical/papers/2003-01-0749http://dx.doi.org/10.4271/2003-01-0749http://www.sae.org/technical/papers/2003-01-1854http://dx.doi.org/10.4271/2011-01-1213http://dx.doi.org/10.4271/2011-01-1221http://dx.doi.org/10.4271/2005-01-0130http://www.sae.org/technical/papers/2005-01-0130http://www.sae.org/technical/papers/2007-01-0219http://www.sae.org/technical/papers/2007-01-0227http://dx.doi.org/10.4271/2007-01-0227mailto:[email protected]://dx.doi.org/10.4271/2011-01-1213http://dx.doi.org/10.4271/2011-01-1221http://www.sae.org/technical/papers/2011-01-1221http://dx.doi.org/10.4271/2005-01-0130http://www.sae.org/technical/papers/2005-01-0130http://dx.doi.org/10.4271/2007-01-0219http://www.sae.org/technical/papers/2007-01-0219http://dx.doi.org/10.4271/2007-01-0227http://www.sae.org/technical/papers/2007-01-0227http://dx.doi.org/10.1243/14680874JER03309http://dx.doi.org/10.4271/2008-01-0016http://www.sae.org/technical/papers/2008-01-0016http://dx.doi.org/10.4271/2003-01-0749http://www.sae.org/technical/papers/2003-01-0749http://dx.doi.org/10.4271/2003-01-1854http://www.sae.org/technical/papers/2003-01-1854http://dx.doi.org/10.4271/2007-01-1285http://www.sae.org/technical/papers/2007-01-1285http://dx.doi.org/10.4271/2009-01-1785http://www.sae.org/technical/papers/2009-01-1785http://dx.doi.org/10.4271/2011-01-0897

7/30/2019 2012-01-1134

13/14

HVA CHARACTERISTICSThe single cylinder engine was equipped with the Sturman Industries HVA system. It has been shown in the body of this paper that

this can be used to evaluate the trends that are shown with different valve timing, lift and duration. However the characteristics lift

profile does differ from a conventional cam and some investigation was carried out to evaluate the possibilities of better-reproducing a

cam based system.

The hydraulic pressure used to control the valve movements on the HVA system is normally set to an optimum (in terms of

response time and stability) which varies depending upon engine speed. The tests carried out here were at fixed engine speed so the

effect of reducing the hydraulic pressure was examined in order to reduce the gradient of the valve opening and closing slope in order

to improve the correlation between HVA and VVA cam profiles.

Figure A1.

Figure A2.

APPENDIX


7/30/2019 2012-01-1134

14/14

The HVA profile along with a 3mm cam profile are shown in figure A1. The hydraulic pressure is reduced from 1700 psi to

1000psi in steps and is also shown in the same figure. It can be seen that for a reduction in pressure, the HVA profile does converge on

the cam profile. However it must be considered that the tail during valve closing is also retarded when the pressure is reduced which

can affect flow through the valve. To illustrate this, the intake air mass flow during motored operation at 800rpm for the same

variation of pressure is shown in figure A2.It can be seen that the mass flow through the valve reduces as the hydraulic pressure is

reduced. At 1300 psi the mass flow starts to increase in this case which correlates with the point when the tail of the valve closing

event is retarding in figure A1 (1000 psi for example). In order to be able to match the cam profile with good accuracy, the valve

duration would have to be reduced to compensate for this effect however further reduction of the HVA valve duration is limited. Dueto the difficulties in reproducing a cam profile with this HVA system, the system optimal hydraulic pressure is used at all operating

points in this paper.


Documents

2012-01-1134