a new way to rate engine power

Max torque @ CFM

Ive been thinking about this for a while now.

All of the ways we have now are quite a missmash of conversions.


Horsepower is derived at after a mulitfactoral calculation, you cant touch it, see it, feel it, or measure it. Using HP for comparison can be confusing when the RPM of one engine is rated at different than the other you are comparing.

this really doesnt say how efficiently the engine uses fuel.



A simple factor of max torque per cfm would facilitate a easy comparison.

The first missmash is every thing is cubic inches of displacement, every two rmps is one full displacement on a 4 stroke. Second missmash is that those cubic inches consume air/fuel at Cubic Feet per Minute.

Theres more too this but lets see how this takes off.

 

Horse Power is sort of like Democracy- not too neat, kind of messy in fact- but it gets the job done in a manner that most of us understand.
The max torque per CFM sounds to me like a solution for which there is no problem.

 

I too, Vicky, have serious questions about "horsepower". It suited James Watt's need the create a standardized rating for low RPM steam engines that "horse" people could relate to. It seems to be of little value in today's short stroke/high revving engines where all HP and Torque curves intersect at 5250! I personally am interested in the torque curve through the designed RPM range of a given engine.
And so, I think you're onto something with your Torque vs CFM formula.
BMEP is also a useful measuring formula - unfortunately it is usually only found on dyno data sheets.

 

No problems ? OK

How or what do you think about;

a small engine (88 Cui) with peak HP of 170 @ 9500 rpm & peak TQ of 95 @ 5500 rpms ( tq curve falls hard @ 7000)

How would be to drive ^^^^

Vs this one

95 cubic inches Peak HP 101 @ 5800 rpm and TQ of 100 @ 4500

 

You can't use displacement x RPM/2 for your calculations.
Picture an engine at 3000 rpm and WOT. It's making lots of torque and power and if it is in a car, it will be accelerating the car with all that extra power.
Now if you close the throttle so it just keeps a steady speed and RPM, it will pump much less air because the throttle restricts the air flow.
Google "Volumetric efficiency" and see if that helps explain why the displacement is not always a direct relationship to power.

 

I guess I don't see the problem you are trying to solve.

And in the example you mentioned in post #4, what transmission? what vehicle weight? what application? street? race track?

Too many variables.

 

Brake Specific Fuel Consumption (BSFC) is a measure of fuel efficiency within a shaft reciprocating engine. It is the rate of fuel consumption divided by the power produced. It may also be thought of as power-specific fuel consumption, for this reason. BSFC allows the fuel efficiency of different reciprocating engines to be directly compared.

http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

 

That's a hyabusa vs harley .

Take a look at the stock factory rated 401 nailhead - tq of 450 @ 2800 rpm
Or a built BBC with 530 @ 4700

 

A great example of a "real" test are the "Nebraska Tractor Tests", which collect a very specific set of data that allows straightforward power and fuel consumption comparisons of different manufacturers tractors. It all started way back in the early 1900's when a fella named William Crozier bought a Ford tractor that didn't perform as advertised. Not many years later he was elected to the Nebraska legislature, and one of the first things he did was establish the Nebraska Tractor Tests in order to keep manufacturers from sell in "junk" tractors in Nebraska. The tests are still used today, and have expanded far beyond the borders of the Husker state.

 

What are you trying to measure? If you want to know how fast you can do work like accelerating a mass, you want power.
Peak power does not depend on rpm which is why the rpm numbers associated with peak power are never the same for different engines.
There are conversion factors to convert HP to any other measure of power.

 

Well the cfm will automatically contain the fuel via fuel ratios.
That the basis for using cfm

The torque is what gets the job done no matter what you are looking at.

High torque at low cfm will get you going faster more efficiently .

Also know this would let an even comparison of effiency be made between a lawn mower and a v12

 

The bsfc method follows the same line but its difficult to understand and follow.
The info isn't readily published or measured but one could easily find the info im talking about on any dyno chart.

How you would use this info is up to you and your particular situation.
For example , if your peak tq and HP is more important than highest average youd be able to rate one combo against another. For me highest average was more important than highest peaks.

 

The volumetric efficiency would automatically be in the calculation and accounted for.

If one engines torque is higher than another and cubic inches are equal and rpms equal then the VE is the difference. Then it would be easy to comparison the differences, is it heads, headers, etc.

 

The 'busa would vaporize the HD in all battles. It has a close ratio 6 speed. The HD has a 5 speed, maybe a 6 speed. They are about as apple to orange as you can get.

 

Remember you want the lowest # s

The Buick would be 1247
The BBC would be 2013

 

The trans and weight of the package aren't part or the engine's power. I'll get the numbers on them in a little bit

 

My brain hurts!!

 

...

 

Its hard to pull your comments out of there to respond ...

Volumetric efficiency really plays a part. If your on top of the game you can actually build in a 5th cycle to an engine. This of course would have a very high VE. But it really doesn't matter when you are looking at things after the fact - its built and is what it is. You'll be compairing A to B.

The CFM reference in the first post is taking on the assumption the goal of the engine is to be more than 100%. Will it be there ? Probably not but that's part of the idea.

 

Just because you have two engines making the same horsepower at different RPM doesn't mean the horsepower ratings aren't comparable. 100 hp at 2000 rpm can always do the same amount of work in the same amount of time as 100 hp at 6000 rpm. For instance, two 100hp cars identical in every way but engine and gearing will go the same speed with the same horsepower if their gearing is proportioned to their engines' RPM rating for the 100hp.

 

True !
However the power of the engine is a completely separate and isolated.

We have MPG that takes everything you just mentioned into consideration and spits out a easily relatable concept.
There's the bsfc for power plants but that info isn't readily available or useful as packaged.

 

How off topic is this???? Go to the SAE website or something,... PLEASE

 

Finn, it was about time someone started taking about BMEP.

Just for comparison:
Naturally aspirated gas engines: 125 to 150 psi.
Supercharged gas engines: 180 to 250 psi.
Turbocharged 4 stroke Diesels: 200 to 270 psi
Top Fuel Dragster: 1150 to 1450 psi.

 

If the brain hurts now, we could throw electric motors into the mix! Off topic? Na, don't think so. I find it hard to believe that there are those (out there) who read every thread posted on a given day and who keep up with additions to previous posts. And so, we individually choose to read what is of personal interest. 24 responses happens to show that there is interest in this type of discussion.
After all, I have yet to experience a hot rod driving on a street or highway that did not have an internal combustion engine providing its motive power. Vicky is not really thinking out side the box: he is taking what is already out there and suggesting a better way of accurately identifying engine output.
No different than Nebraska Tractor tests!

 

You don't see BSFC curves on most published dyno reports, but it's actually easier to measure than CFM; because the dyno can take readings from a liquid flowmeter. Many induction systems don't lend themselves to direct CFM monitoring, and some builders I've talked to feel the fan type airflow meters can actually affect the breathing of the engine.

On of the top west coast engine guys in the division I used to race in swore by BSFC as a measure of efficiency. I'm pretty sure NASCAR is very interested in BSFC info as well, trying to stretch them pit stops...

 

Horsepower has specific units to measure how much work an engine can do.

Force*distance/time

Suppose you need to move 10,000 lbs up a 30% grade (16.7° angle)

If you have 100 hp, you can move it at 13 mph. To move it 60 mph requires 460 hp. It does require math, but it's pretty straightforward.

What you are looking for in your first post isn't power, it's BSFC, as previously mentioned.

 

For as long as I can remember engines have been referred to as an air pump.
OK - If that's a bad anology that's been published countless times , repeated and taught to millions - well then ignore this statement of the obvious. Air pump's purpose is to pump air and generation of air flow & that airflow is measured in CFM. Obvious right ? The more CFM the more FUEL. For gasoline engines, the stoichiometric, A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel.

OK so there's the Cold hard facts and the empirically obvious.

The air pump in question will have a displacement and vary from one configuration to another. For example look to the SBCs 302, 305, 307 the Pontiac 301 and olds 307 very small percentage (no more than 2%) or the Chevy 427 short 427 tall 427 ford, 428 ford, 426 hemi, 428 Pontiac again such a small percentage in displacement. They all will have different performance in stock form and who knows on custom builds and bolt ons.

If you were to look at a close group at say 3000 rpms , the theoretical calculated cfm and related fuel would result in predicted level of power by the amount of energy in a pound of fuel. Your calculations on power output would be way off on the dyno. We could do this measurement at the entire rpm range too. These differences would be due to VE, bore to stroke ratios, rod length, and rotational friction blah blah.

One of these combos will result in the highest torque (read power output) at less CFM ( read RPM). That would be max torque @ CFM.

Once that's converted to some ratio or unitized factor, one could use that factor to across other combos.

There's more but in the intrest of not being long winded I'll stop here for now

 

BSCF is a unit of fuel consumption if I understand it correctly.
I'm looking for MAX power or torque against the least amount of fuel used with a way to easily compare two different displacements at two different max torque RPMs
Perhaps its in the BSCF and only requires a string of calculations or specialized search engine to extrapolate the info.

 

I think the basic problem you're going to encounter is not just explaining why there is any meaning in a "max torque @ CFM" parameter, but in measuring CFM. It looks to me that you are saying, "For CFM read RPM", meaning they are directly convertable. If I read you correctlly, you don't like horsepower as a key parameter because it's not "directly" measured (although it's still no more "calculated" than torque is because HP measurement justs adds a counter and a clock to the force and length measurement of torque). So how do you propose to measure CFM?