dynosim question
 

i was running my engine build on a "desktop dyno" program called dynosim, and it gave me some numbers that didnt make sense to me. it said that for my given engine if i were to run it on gasoline it would make 630 ft/lbs and 580 horse, on ethanol 675 ft/lbs and 600 horse, and on propane 580 ft/lbs and 550 horse. what i dont get is that propane has a higher energy density than ethanol, as does gasoline... so why would ethanol give better numbers?

Energy content is only one part of the equation. Ethanol (or E85) has a higher octane rating, so you can increase the compression and/or advance the timing more then with gasoline, and both of these can increase performance. That's one possible explanation, but there may be more. I don't know dynosim, but there's always a possibility that you get a wrong answer from a simulation program.

the engine was identical for all simulations. identical head flow, identical intake flow, identical cam, identical compression, identical displacement. the only difference was the fuel, even the curves were identical... just upshifted or downshifted for the fuel... the only reason i can think of is maybe because ethanol is a "partially oxygenated" fuel so it would in theory burn more easily or at least take less air to burn thoroughly... but i wouldnt be able to use it because of its crappy cold start characteristics. after all, i live in north dakota- lots of cold weather starting, and i cant afford race gas all the time (just shy of 13-1 compression) so i picked propane for the fuel system. i just changed the fuel for fun. the ethanol numbers were about what i was expecting to see for all fuels (at least approximately) but regardless- man what a truck engine :)

There are two main reasons you saw the numbers you did. One, is latent heat of vaporization. Ethanol cools the air a lot more as it evaporates than gasoline. Cooler denser charge = more power. Two, is even though the energy content per amount of ethanol is lower than gasoline, the stoichiometric ratio is also lower. So you're using more of it to burn the oxygen in the air. So, even though the energy content is lower, more of it is going into the cylinder, and if you figure it up you come out slightly ahead in terms of energy per combustion event.

Propane is already vaporized, so it doesn't cool the intake charge at all. Now if someone found a way to inject liquid propane... }>

Something else to note is that the oxygen in the ethanol molecule is not available for burning. It, in a sense, has already been burned. It is just along for the ride, so to speak. This is why the stoichiometric ratio is lower than gasoline, not higher.

Also note that the higher compression ratio increases the efficiency of the engine. Where ethanol has an advantage, as already stated, is it allows you to run higher compression ratios. So even though ethanol has a lower energy content, you can extract more of it per unit of ethanol if you run higher compression than you can gasoline. To what extent I'm not sure though.

If it's a free or relatively low-cost program, it may be in the way it's coded, or it's accuracy. Ethanol, gas, and propane are not meant to be run identically in the same engine. The way the program is coded might be reading the input values differently for each, and giving you those skewed values back. Also, the values are within about 5% of each other, which is about right for a free simulator.

And I agree-that is one heck of an engine :-D . What engine is it?

its for my 545 ci 460 stroker project... blue thunder heads, hydraulic roller retrofit cam, dual quad intake, 1.8 ratio stainless rockers, titanium retainers, 12.89 static compression (just under 9.5 dynamic) running dual impco 425 propane carbs
also i spent a hundred dollars on the program, and that was on sale- down from 260... if i hadnt paid so much for it i wouldnt be worried about the numbers it puts out

The numbers sound exactly where they should be. Nothing wrong with the program.

Nice engine!

As bad as it sounds, that is a relatively low-cost program. The professional level programs are over $1000, so the numbers it's putting out still don't surprise me. Try playing around with some of the settings, and make the engine more "realistic" for ethanol. Changing to a higher or lower compression ratio might be able to give you something more along the lines of what you were expecting. Also, try running gas & propane with the different numbers, that'll at least give you a benchmark as to how consistent the number outputs for the program are.

And damn you, you're just giving me another reason to spend money on starting up a 545 project.