So, this is somewhat related to the "Hemi" thread in that engine efficiency was mentioned, but touched upon very little. Let's try to clear up some confusion.(or cuase some perhaps? [&:])

First of all, measuring horsepower relative to engine displacement is a very poor standard for comparing engines. That's due to the fact that horsepower output is highly dependant upon both engine rpm and displacement. To get a better comparison you'd have to measure engine rpm vs displacement vs horsepower output. Since horsepower is dealing with torque and rpm you can throw out both horsepower and rpm and be left with torque(torque is what torque is in an engine, whereas horsepower is derived from known torque and known rpm). This leaves us with comparing engines by torque vs displacement. This actually generates the quite handy BMEP or brake mean effective pressure, which is a measurement of the mean pressure that would need to be exerted across the surface of the piston for the entire length of the stroke to generate the given amount of torque per cubic inch. It's quite handy since you only need peak torque and displacement, and it cleans up dealing with RPM/horsepower. Now, it should be noted that in an engine combustion takes place rapidly, and the same pressure is not exerted for the whole length of the stroke, in actuallity most of the work is already done by the time only half the stroke has occured. Generally combustion peaks suddenly, sometimes to several thousand PSI(depends greatly on the engine setup) of pressure very briefly, then drops off dramatically. But, BMEP though a theoretical calculation, compares the averages of combustion pressure of differing engines.

It takes 150.8psi of BMEP to generate 1lb-ft of torque per cubic inch. So we end up with BMEP = torque x 150.8 / CID. In this way we can compare average combustion pressures(combustion pressure being the true measure of how well an engine can produce power relative to it's size) of differing engines. You could have 2 engines with the same BMEP, but one is 2x as large so it produces 2x the torque. The BMEP is the same, so in that regard we could say they are equally as efficient in terms of combustion pressure.



The other measure we can use was touched upon in the other thread, and that is thermal efficiency, which is what physicists would use when talking about the efficiency of 2 mechanical devices. Any machine, simply put, takes some source of potential energy, converts some of it to mechanical energy and loses the rest primarily as heat energy. An automobile engine takes fuel as potential energy, produces usable mechanical energy that we measure as torque/horsepowerm and loses the rest mostly as heat. The more usable work that can be generated from the same amount of fuel, with less heat loss, the more thermally efficient the device is. For car engines this is expressed as BSFC, or brake specific fuel consumption. It's a measure of how much horsepower that is produced in an engine vs the fuel consumption per hour to do so. It disregards rpm and torque entirely, and uses only horsepower(the measure of how much work can be done in a given time, in this case the 1 hour standard used to measure fuel flow) and fuel consumption to determine efficiency.

It's expressed as BSFC = fuel flow(in lbs per hour) / horsepower. By this we can compare 2 engines of any size that make their power at any rpm if we know peak horsepower and fuel consumption. The idea is to look at thermal efficiency, how much usable work can be generated from a given amount of fuel. For instance, we could have 2 engines that make the same horsepower, only 1 is larger in displacement, but they both consume the same amount of fuel to do it, they would both be equally as thermally efficient...producing the same amount of usable work in the same amount of time from the same amount of fuel.


So, when we talk about efficiency we should clarify just what we mean by that, whether we're talking about thermal efficiency, or whether we mean combustion efficiency(cylinder pressure).

If we wanted to try to lump everything together to a degree, we would end up with volumetric efficiency, which is a measure of thermal efficiency vs horsepower vs displacement vs rpm, including a constant(9411). This is expressed as VE = (9411 x hp x BSFC) / (CID x rpm). VE is only a useful comparison in 2 engines of the same induction type, n/a or f/i. Comparing VE's of n/a vs f/i engines is extremely lopsided, since BSFC is still nearly the same(it could be slightly higher in a blown application where a richer mixture is chosen to help supress detonation) but horsepower is higher with the same relative engine size and rpm.

Just some interesting info on all this, if we look at some of the most well developed n/a powerplants in the world, Pro Stock drag cars, Nextel Cup cars and Formula 1.....

They all operate with high thermal efficiency, in the neighborhood of around .44 give or take(the ones that are tuned the best). They all produce similar BMEP, in the 220-230psi range, some of the best possibly breaking into the 230-240psi range. Nextel Cup engines(without a restrictor plate) and Pro Stock engines produce slightly higher BMEP than the Formula 1 engines with the nod most likely going to the Pro Stockers, perhaps exceeding 240psi at times. Formula 1 engines aren't marvels of torque production, but trade torque for horsepower in their rediclous rpm ranges(upwards of 20,000rpm). VE for the Nextel Cup engines and Pro Stockers tends to run upwards of 110-115% or more. Formula 1 engines tend to run VE's in the neighborhood of 125-130% or more. To put it another way, in order for a 5.0L engine to have a VE of say 127% at 7,000rpm, it'd need to make around 650hp.

As a side note, the induction velocity in the head ports on F1 engines is super sonic.

 

Excellent write-up. Some very good and highly detailed points were brought up.