P. Bauer and J. Murillo

University of Notre Dame

Dept. of Electrical Engineering

Notre Dame, IN 46556, USA

Biodiesel versus diesel: A comparative analysis of the effect of engine cycling on

efficiency

TABLE OF CONTENTS

1 - Introduction

Preliminaries and Concepts

2 -Preliminaries

BSFC

Cycling induced BSFC

BioDiesel bsfc

3 -Analytical Results

4. -Conclusions

5. -Future Research

1. INTRODUCTION Hybridization of powertrain: fuel

savings due to avoidance of high bsfc operating regions in ICE

Our focus: Large ICEs (Diesels) in a series hybrid configuration

Large ICEs in a series hybrid powertrain It is not possible to cycle between the bsfc optimal power and the engine off state

Important question: Do the advantages of cycling carry over from Diesel to Bio-Diesel ?

Applications areas: large trucks, earth moving equipment, locomotives, Diesel generators, etc.

Hardware requirements: ICE, generator, energy buffer, inverter, e-motor(s), mechanical powertrain, engine controller.

Diesel Engine

Generator AC

Fuel

Rectifier DC Battery Load

2. PRELIMINARIES AND DEFINITIONS Assume the engine will operate at discrete

power levels , in this case two power levels

Denote the two engine operating points as , .

Define the minimal achievable brake specific fuel consumption at an engine power level as

Denote the power level of the global minimum as and the low power OP as

Typical full load BSFC ranges:

In order to illustrate the concept we will use some artificially generated bsfc curves

2. PRELIMINARIES AND DEFINITIONS

Original minimally achievable engine BSFC for Diesel

Cycling induced engine for Diesel

Minimally achievable engine BSFC for Bio-Diesel

Cycling induced engine BSFC for Bio-Diesel

The difference between BSFC for Diesel and Bio-Diesel as function of brake

power

: Engine brake power

: Brake power for which the BSFC minimum is reached for Diesel

: Power of low power operating point for Diesel

(2. Cont.) CYCLING: Basic Concepts Fuel savings can be achieved by cycling in a

series hybrid powertrain

The engine is operated at two discrete

operating points, and

Average power produced is between operating point power levels and must meet power output demands

Excess power generated at high engine power levels can be stored in an energy buffer

Stored power can be pulled from the energy buffer to compensate low engine power levels

Our analysis is asymptotic, i.e. for large cycle periods and large energy buffer size

Cycling between and

(2. Cont.) CYCLING: Basic Equations

Fuel mass, M, consumed at a power, P, over time T

Average Power, , as a function of time for and

Mass of fuel consumed cycling between P1 and P2

𝑀=𝑃 ∙𝑇 ∙𝑏𝑠𝑓𝑐 (𝑃 )

𝑃𝑎𝑣𝑔=𝛼1∙𝑃1+𝛼2 ∙𝑃2𝑎𝑛𝑑𝑃1<𝑃𝑎𝑣𝑔<𝑃2

𝑀 𝑐𝑦𝑐𝑙𝑒=𝑇 ∙𝑃1 ∙𝑏𝑠𝑓𝑐 (𝑃1 )+(1−𝑇 ) ∙𝑃2∙𝑏𝑠𝑓𝑐 (𝑃2 )

(2. Cont.) THE CYCLING INDUCED EFFECTIVE 𝒃𝒔𝒇𝒄

A new bsfc curve generated under the assumption that an engine will be cycled between two operating points

low power operating point

at the global bsfc minimum power level

Cycling induced fuel consumption often better than fuel consumption under regular operation.

𝑏𝑠𝑓𝑐 (𝑃𝑎𝑣𝑔 )=𝑃1 ∙ (𝑃𝑎𝑣𝑔−𝑃𝑜𝑝𝑡 ) ∙𝑏𝑠𝑓𝑐 (𝑃1 )+𝑃𝑜𝑝𝑡 ∙(𝑃1−𝑃𝑎𝑣𝑔) ∙𝑏𝑠𝑓𝑐 (𝑃𝑜𝑝𝑡)

𝑃𝑎𝑣𝑔 ∙(𝑃1− 𝑃𝑜𝑝𝑡)

(2. Cont.) A REALISTIC CASE: The Cummins B-Series EQB235-20 Diesel Engine

Fuel savings are possible in real, existing engines

Key characteristic: the “flatting

out” of the engine bsfc curve

For larger engines, greater margins of fuel savings are

possible

(2. Cont.) Bio-Diesel BSFC

Relationship between , bsfc(P) and .

Relationship between curves for regular Diesel

and Bio-Diesel, with the difference between the

two being denoted as . For practically all Bio-

Diesel fuels and mixtures with regular Diesel, the

efficiency drops relative to regular Diesel, i.e. the

increases:

Often is approximately constant and not a strong

function of

3. ANALYTICAL RESULTS

, (1)

The bsfc values for Bio-Diesel is always higher than that of Diesel.

The difference between Diesel and Bio-Diesel bsfc depends on the

BioDiesel mixture

Usually the difference between Diesel and Bio-Diesel bsfc is only a weak

function of power and can often be considered constant over certain

power bands

(3. Cont.) ANALYTICAL RESULTS

(2)

Relationship between induced BSFC for Diesel and Bio-Diesel

The induced BSFC for Bio-Diesel is always higher than that for Diesel

The difference in the induced BSFC depends on duty cycle and the difference in

BSFC values at power and

(3. Cont.) ANALYTICAL RESULTS

(3)

Condition for cycling induced BSFC (for Bio-Diesel) to be lower (better) than

the regular BSFC (for Bio-Diesel)

If the inequality is true, cycling is advantageous

(3. Cont.) ANALYTICAL RESULTS - Key Result

(4)

Condition for cycling to be advantageous

If the BSFC difference between Diesel and Bio-Diesel is large in a certain

power band between and , and small at and , cycling is preferable.

(5)

This condition is often approximately satisfied, i.e. the BSFC difference between Diesel and BIO-Diesel is approximately constant over certain power bands

With the previous equation (4) this shows that in this case the advantageous of cycling carry over from Diesel to BioDiesel

A notable case where these advantageous may not carry over is if the BSFC difference is high at P1 and Popt and low in between.

(3. Cont.) ANALYTICAL RESULTS

(3. Cont.) ANALYTICAL RESULTS – Example BSFC by S. Savariraj et al.

(3. Cont.) ANALYTICAL RESULTS – Example BSFC by S. Savariraj et al.

4. CONCLUSION

This paper provided an analytical efficiency comparison of cycling

operations with Diesel and Bio-Diesel.

Conditions were derived that ensured that the efficiency advantages of

cycling for Diesel carry over to Bio-Diesel.

It was shown that in most cases these advantages carry over to Bio-Diesel if

the same operating points are used.

However it is currently not clear, if significant improvements in efficiency

are obtainable if the operating points are changed when transitioning to

Bio-Diesel.

5. FUTURE RESEARCH

(1) Changing operating points to maximally exploit the Bio-Diesel BSFC –

How much can be gained?

(2) Emission effects of frequent operating point changes

(3) Does flattening out of the bsfc curve reduce the benefits of cycling in

the case of Bio-Diesel?

THANK YOU!!!