Pent roof combustion chambers and minds

[C M Smith <cmsmith@domain.elided>]

A recent post appearing to rely upon what the "best engine minds" do about 
combustion chamber shape draws one (at least) unwarranted conclusion. 
Assuming the writer intended to refer to racing engine designers, and of F1 
engines at that, then it is correct that the pent roof is the ideal shape, 
as Keith Duckworth so cleverly demonstrated all those years ago with the 
Cosworth DFV. It is incorrect to deduce that this is because the shape is 
ideal, far from it.

It is important to remember that these engines have extreme bore/stroke 
ratios which seriously limits your options for combustion chamber shape, if 
high compression is desired, which it uniformly is for non turbo engines. 
The extreme bore/stroke ratio results from the simple geometry of total 
swept volume and maximizing piston area within that limit. A pent roof was 
essentially the only way to go, when Duckworth crunched the numbers.

It is true that larger valves slow down the intake flow, but only through 
the valve. The speed of the intake flow in the inlet pipe goes up, which is 
more to the point. The cylinder filling resulting from intake effects is as 
much about resonance as it is about inlet tract to valve port angle or 
valve area or lift. Bigger valves breathe better, all other things being 
equal. More precisely, bigger total valve opening area breathes better AOTBE.

A hemi head design still has advantages over pent roof for some 
applications. Not all engines can run at 18,000 rpm or head blowing 
compression ratios. The undersquare Alfa four is an example. The 164 engine 
also uses hemi heads in the two valve version even with a healthy 
oversquare engine. Bigger valves are possible with the hemi design, but 
multiple valves don't fit so well for reasons of actuation complexity. Pent 
roof accommodates multiple valves in an arrangement relatively easy to 
actuate with ordinary valve gear. Porcupine heads have been tried with some 
success in the past but they are complex, and who knows, maybe the valve 
actuation work being done now will result in a return to a semi hemi?

The best minds in engine design agree on one thing: it depends...

Cheers


Michael Smith
Calgary, Alberta,Canada
91 Alfa 164L
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