Gasoline contains a finite amount of energy. How much horsepower an engine can generate depends on how effectively it can turn the energy stored in the gasoline into power in the crankshaft. How well it does this depends on a number of variables:
how well and effectively it can provide the optimum fuel mix for each cylinder as it runs
how readily it can fill the cylinders with that fuel mix as the pistons go down and draw it in
how effectively it can combust the fuel. This goes to cylinder design (wavefront propogation) and ignition system design
how effectively it can convert that combustion to turning force in the crank. This goes primarily to piston and cylinder design, compression, crank length (stroke length)
how much of that energy it can use to turn the tranny shaft as opposed to using it up in its own operation. High reciprocating masses and long strokes “waste” more of the energy. Mechanical lifters use more energy to operate than roller bearing lifters. Pushrod valvetrains tend to be less efficient that overhad cam valvetrains, especially when the RPMs get high and the valves “float” (don’t fully close).
how effectively it can expell the used up exhaust gasses. That’s why headers add more horsepower
and, lastly, how much of the fuel it can actually get into the cylinders if everything else were at optimum…that is the displacement or “engine size”.
A larger engine can theoretically be made to potentially produce more horsepower because it simply has more capacity in the cylinders, however size is only one in a number of critical variables, each of which plays its part. A large engine with large reciprocating masses and poor ability to breath in and out and poor ability to effectively use the energy in the gasoline can have much less horse power than a smaller engine that is optimized in the listed aspects.
So, it simply isn’t that simple. Size does not necessarily equal power.
how effectively it can combust the fuel. This goes to cylinder design (wavefront propogation) and ignition system design
Don’t forget valve inefficiencies like overlap.
Mechanical lifters use more energy to operate than roller bearing lifters.
Both flat tappet and roller lifters are available as “mechanical” or solid lifters. Roller bearing lifters are more “efficient” than flat tappets. Is that what you meant or did I misunderstand?
and, lastly, how much of the fuel it can actually get into the cylinders if everything else were at optimum…that is the displacement or “engine size”.
Mass efficiency or “dynamic compression ratio” are two other ways to describe it.
It’s always amazing to look back and see what was accomplished in the very early years and really how little we have actually progressed. Especially considering the tools (or lack thereof) those engineers had to work with back then.
I guess that must have been one of the Grand Prix racing supercharged engines. Progess continues, today’s 2.4L engines in Formula 1 put out close to 800 hp. Racing engines have alway put out much more hp/L than street engines.
Yeah, I sort of lumped that under “cylinder design”. Perhaps that was inappropriate. I was trying not to get too deep.
You understood. Actually, I was trying to think of “lay” ways to word things that would avoid the discussion of hydraulic vs. mechanical, ways to describe the reduced wasted energy of using bearings rather than flat tappets. I also intentionally avoided the subject of possble enhanced cam profiles possible with roller lifters and avoided the subject of valve spring rates and such.
One big difference that I “assumed” under the whole description of how effectively an engine can combust fuel is the differences between carburation, throlle body injection, and multiport injection. The whole surface-area-per-volume enhancement of a fine mist in a tailored spray pattern shot right into the intake port at exectly the right time is a big plus.
I skipped the discussion of boosted pressures and their place in the horsepower equation also.
And I struggled with whether to say “stroke length”…I gagged when I wrote it but I figured it would make more sense to the layman than just “stroke”.
It found it hard to put into a simple explanation why more CID doesn’t automatically mean more HP. But I think I got the idea across.
It’s a testament to the genius of the early pioneers.
We’ve made tremendous progress in the areas of the manufacturing and design technologies, enabling far smoother, more reliable, and more efficient engines, and in fuel delivery systems (I’m including the sensors and computers in that statement), but by and large engines run pretty much the same way they did in the early years. With the exception of the Wenkle.
Hybrids are, IMHO, over rated. As are fuel cell technologes. I suspect the next real greakthrough is going to be in battery technology. Some new way of storing energy in extremely dense packages. Something beyond lithium ion.
The tradeoffs with race engines being cost, reliability, and longevity. Purpose specific designs will always outperform designs that try to please the masses.