1. Design objectivesThere are many different parts of an
engines design that controlthe amount of power you can extract from
each combustion stroke.For example: You want to burn all of the gas
in the cylinder. If the designleaves any of the gas unburned, that
is untapped energy. You want the maximum cylinder pressure to occur
when thecrankshaft is at the right angle, so that you extract all
of theenergy from the pressure. You want to waste as little of the
engines energy as possiblesucking air and fuel into the combustion
chamber and pushingexhaust out. You want to lose as little heat as
possible to the heads and thecylinder walls. Heat is one of the
things creating pressure in thecylinder, so lost heat means lower
peak pressures.
2. new designs from Chrysler The years following World War II
brought upon new designs forperformance, Chrysler had to find a way
to increase power withoutincreasing compression, which would
require higher octane fuel. The key was thought to be in the better
designed cylinder head,perhaps hemispherical with conventional
valve-in-head. Using hemi heads would increase thermal and
volumetric efficiency, aswell as provide a low surface-to-volume
ratio (thus minimizes loss dueto combustion-chamber deposits).
Complexity and high costs did not allow easy mass production,
andthis rugged design loved high octane gas. Chrysler teams
researchedevery engine available, and the hemi head design proved
to be the mostpowerful and efficient model they could find.
3. Introducing HEMI
4. the HEMI head versus the flathead
5. Cylinder headHEMIFLATHEAT
6. pistonHEMI FLATHEAD
7. Construction drawing
8. combustion in Hemi Engine
9. valves A hemispherical combustion chamber allowsthe valves
of a two-valves-per-cylinder engine to faceeach other across the
chamber, rather than openingside-by-side. This layout makes space
in the combustion chamberroof for larger valves and straightens the
airflowpassages through the cylinder head. This creates what is
known as a cross-flow head,where the intake charge flows directly
across thechamber to the exhaust valve located directly oppositeit.
which adds efficiency and power
10. A hemispherical engine spreads the valves apart and places
them at an angle todecrease resistance, but its only an improvement
on a crossflow head. Thesefeatures significantly improve the
engines airflow capacity, which can result inrelatively high power
output from a given piston displacement. But the design can also
significantly increase the flow of incompletelycombusted air-fuel
mixture straight out of the exhaust valve. With a hemicombustion
chamber, there is minimal quench and swirl to burn the fuel-airmix
thoroughly and quickly The hemi head usually has intake and exhaust
valve stems that point indifferent directions, requiring a large,
wide cylinder head and complex rockerarm geometry in both
cam-in-block and overhead cam engines. This adds tothe overall
width of the engine, limiting the vehicles in which it can
beinstalled.
11. Valve Train The hemi motors tappets and pushrods are
accurately pointed at the rocker arm ends for best mechanical
efficiency. The intake and exhaust rocker arms are different in
length, and pivot in opposite directions. The valve seats are
located directly across the head from each other and 90 to the bank
axis, not parallel to the cylinder bank axis
12. Spark plug the spark plug is frequently placed at or near
the centroidof the chamber to facilitate complete combustion.
Hemispherical combustion chambers, because of their lackof quench
they are more sensitive to fuel octane rating. a given compression
ratio will require a higher octanerating to avoid ping in a hemi
engine than in a wedgeengine. Engines with hemispherical combustion
chambers oftenuse dome-topped pistons to attain the desired
compressionratio but this design only works best at high
R.P.M.
13. Pistons All original pistons are cast aluminum alloy. Most
have slight compression domes (area of slightly smaller diameter
than the bore) with two valve clearance notches. All have floating
piston pins with retaining clips.
14. HEMIAdvantages
15. Surface area Surface area causes heat loss. Fuel that is
near thehead walls may be so cool that it does not burnefficiently.
With a flat head, the amount of surface area relativeto volume of
the combustion chamber is large. In a HEMI engine, the surface area
is much smallerthan in a flat head, so less heat escapes and
peakpressure can be higher.
16. size of the valves Since the valves are on opposite sides
of the head,there is more room for valves. The engine design
thatpreceded the HEMI was a wedge-shaped combustionchamber with the
valves in line with each other. Theinline arrangement limited valve
size. In a HEMI engine, valves can be large so the airflowthrough
the engine is improved.
17. Spark plugs Spark plugs were located in the center of the
cylinder. This created a very even flame front, increasing
combustion efficiency (often offset, however, by large domed
pistons which shrouded the spark plugs)
18. compression ratio Because of the above advantages, Hemi
engines could run higher compression ratios without the fear of
detonation.
19. HEMIdisadvantages
20. HEMI Disadvantages If HEMI engines have all these
advantages, why arent all engines using hemispherical heads? Its
because there are even better configurations available today.
21. valves per cylinder Having only two valves per cylinder is
not an issue indrag racing or NASCAR because racing engines
arelimited to two valves per cylinder in these categories,But on
the street, four slightly smaller valves let anengine breathe
easier than two large valves. One thing that a hemispherical head
will never haveis four valves per cylinder. The valve angles would
be so crazy that the headwould be nearly impossible to design.
Modern engines use a pentroof design toaccommodate four
valves.
22. combustion chamber size Another reason most
high-performance engines nolonger use a HEMI design is the desire
to create asmaller combustion chamber. Small chambers further
reduce the heat lost duringcombustion, and also shorten the
distance the flamefront must travel during combustion. The
compactpentroof design is helpful here, as well.
23. high RPM range The big disadvantage is in the really high
RPM range. With the valves opposing each other, cylinder pressure
at really high RPMs is lost on the overlap cycle of the cam. At
lower RPMs, say 7500 or less, the Hemi is unbeatable.
24. Fuel This engine is very senstive for octane number of fuel
This engine has high rate of fuel consumption
25. Expensive This engine is expensive in production due to
it`s complex design
26. Development of hemi engine Due to this limititions,
developers introduced new modification in this design. That`s
called a pent roof design Now , we will show how this new design
overcome the limititions of the previous one
27. Construction drawing
28. THE SHAPEHEMIPentroof the combustion chamber is the
combustion chamber issemi-spherical. like a triangle.
29. THE HEAT EFFICEINCYHEMI Pentroof they were created to be
more it is more efficient at keepingeffiecent at keeping heat
forheat in and has a smallerbetter combustion and more surface-area
to volume ratiocomplete buring of the fuel in than the hemi.the
combustion chamber.
30. CYLINDER HEADHEMI Pentroof It was created to have a valve
Improved upon the HEMIon each side of the(of-course, its newer) and
putcombustion chamber/ usuallytwo valves for intake and twowith the
spark plugin in the for exhaust for better flow.center. Because of
the domeshape it can only have 1 valvefor intake and one
forexhaust, the valve train wouldbe too complex suposedly.
31. Hemi engine in the past Chrysler IV-2220 fighter plane
engine Chryslers first hemispherical-head engine was the IV-2220,a
2,500 horsepower, supercharged, turbocharged V-16.Chrysler had
begun work on the engine around 1940, andpresented their idea to
the United States Army Air Corps, aWorld War II era predecessor to
the modern-day US AirForce. The engine was intended to be installed
in RepublicP-47 fighter planes. It first flew in 1945, but it never
wentinto production. By that time, the war was ending, and theneed
for such an engine was dwindling as the jet ageapproached. One of
these engines is on display at the NewEngland Air Museum in Bradley
Airport, Windsor Locks,CT.
32. HEMI engines nowdays
33. The 2003 Hemi ModelTodays newest HEMI engines build off the
original designs of the firstHEMI engine.The new models still
incorporate the hemispherical head and pridethemselves on being the
one of the most powerful and bestperformance engines in the
world.The 2003 Dodge HEMI carries on the tradition of HEMI engine
andproduces more power than any other listed engine and can also
run at ahigher rpm than other motors. Manufacturer Size (Liters) #
of Cylinders Horsepower RPMDodge 5.7 8 345 5400Dodge 8.010 305
4000GMC 6.0 8 300 4400GMC 8.1 8 340 4200Ford5.4 8 260 4500Ford6.810
310 4250
34. Companies use HEMI Chraysler Dodge Porsche Ford Aston
Martin Mitsubishi Alfa Romeo Jaguar Lotus
