High compression pistons=more HP=lower gas mileage??????????????

[DamnedButDetermined]

I am stuck in the middle on this one. Normal thought would say the higher your horsepower the lower your gas mileage. But in the case of high compression pistons all you are doing is compressing the same amount of air as before into a smaller space. This does increase the efficiency of the burn and therefore creates more power.

So does this mean lower gas mileage?

[Jeff240sx]

Fuel economy is directly related to horsepower and tuning, hence the brake specific fuel consumption formula.
-Jeff

[Maeda]

i'm thinking in general more air means more combustable oxygen which leads to more engine heat... therefor requiring more fuel...
anyone feel free to correct me but i think if you do high compression pistons youd need more fuel... kind of like raising the compression in forced induction applications right?

[DamnedButDetermined]

you see but i forced induction you are adding more air, with high compression pistons you are using the same amount of air as before, but you are compressing it into a smaller space! That is why i don't see the need for more fuel. You will have to use higher octane fuel (b/c the more you compress something the hotter it gets, PV/T, and if you continue to use 87 octane it will detinate prematurely).

Jeff240sx, what is this brake specific fuel consumption formula?

[Jeff240sx]

lbs/hr = (BHP × BSFC) ÷ (No. of Injectors × DC)

Basically, fuel requirement = Brake (crank) horsepower x Brake Specific Fuel Consumption (semi-constant) ÷ # of injectors x Duty Cycle.
The BSFC is usually .4-.5 on NA engines, and .6 or so on turbocharged engines.
You can see that if the hp increases, the lbs/hr requirement increases.
-Jeff

[DamnedButDetermined]

wow that was like the concord just flew over my head!

Do you understand were i am comming from though? It just doesn't make sense to me why th gas mileage would deteriorate, just because you are compressing the same amount of air as before, into a smaller compartment.

Sorry to ask you to do this, but can you explain your reasoning in laymans terms?

thanks.
-DbD-

[Jeff240sx]

#/hr = (hp x .45) / 3.2

As you can see, if hp goes up, fuel requirement goes up.
-Jeff

[The Riot Hero]

i disagree, newer cars are being made with hgih compression to make power and still keep good gas mileage, anything that gives the car more horsepower is making it more efficient, more efficeint equals ebtter gas mileage. the only reason i coudl think it woudl need more fuel is to keep exhaust temps down so the car doesnt detonate.

[d240t2]

Originally posted by Jeff240sx
#/hr = (hp x .45) / 3.2


That is all fine and good...if you are doing mods that keep the BSFC the same. Raising the CR lowers the BSFC. I am not sure how that effects gas mileage offhand (if the lowering in BSFC with the raise in hp requires that you use more fuel or less), but I don't claim to know for sure. Honestly, I think raising the compression gives you a more efficient engine, which is what gives the higher hp number. I would be surprised if you get any significant changes in gas mileage (more significant than the effect your driving style has). I am pretty sure it would actually raise gas mileage...since it gives you a more efficient engine.

I think you just took the wrong equation, took it out of context, and then held a variable constant that is definitely something you are changing.

Dennis

[mrdirty]

OK, someone for sure will $hit on me for giving a tech answer but anyway:

-Efficiency (thermodynamic) is the ratio: energy in/work out
-Efficiency increases when the difference in pressure (atmospheric to compressed) goes up
-As efficiency increases, either fuel consumption goes down or power goes up.
-Increasing compression (via taller pistons) decreases the amount of air going into the engine, thereby decreasing fuel consumption; the difference in pressure from TDC to BDC more than compensates for this.

So, higher compression leads to higher engine efficiency.

Why don't we compress to just shy of detonation?
Because the fluid (air) dynamics are very complex in an engine at high rpm, a pressure gradient forms at the piston on the compression stroke which would detonate a fuel mixture earlier than expected (which is bad), and also air and gas are not perfect (outside a lab) so we have to design for the worst cas (bad gas, crappy air).

This is why direct injection is the $hit, we can compress that air like a mofo and THEN dump in the fuel just ahead of the flame front, getting monster compression and thereby monster efficiency.

I'll stop here unless anyone wants more details...

[Jeff240sx]

Originally posted by d240t2
Raising the CR lowers the BSFC. I am not sure how that effects gas mileage offhand (if the lowering in BSFC with the raise in hp requires that you use more fuel or less)
I think you just took the wrong equation, took it out of context, and then held a variable constant that is definitely something you are changing.

Dennis

You're right! I neglected to think about the efficiency going up as the hp went up. Now the question is if the 2 points of compression increase changes the hp more than it will change the bsfc.
Also... for the second part, I probably did do all you mentioned, but if you look 4 posts above yours, I gave the correct formula, stating it was a semi-constant. The post you quoted was the "watered down, easy to explain" version for DbD.
-Jeff

[DamnedButDetermined]

very nice explainations everybody! I kind of got the equation Jeff240sx put up, and everyone else confirmed what i was thinking.

Do we have direct ignition? I don't think so but worth asking.
What cars do have this?

So this is what i thought in the beginning. Engine efficiency would go up, and gas mileage would remain the same as long as driving habits remain the same.

this was a very well befhaved debate...I am very proub of you Zilvians

-DbD-

[mrdirty]

Originally posted by DamnedButDetermined
gas mileage would remain the same as long as driving habits remain the same....

If the air/fuel mixture is right on then gas consumption will drop.

[mrdirty]

Originally posted by DamnedButDetermined
...
Do we have direct ignition?...

u mean direct injection? No, few companies have been able to get it right, it needs very high fuel pressure and high flowing accurate solonoids.

I think most of the major companies have an engine in development w/ this technology.

[edspecR]

if im not mistaken....doesn't dfi stand for direct fuel injection? so...i dunno, but dont a couple of companies already have dfi systems?

[ygblat]

as far as i know, the new BMW v12 is the only engine on the market that utilizes direct injection. the differences are not really that big between it and regular fuel injection. the injector is now in the actual combustion chamber so it allows higher pressure injection and lets the injectors squirt finer mists of fuel. also, the injector now functions independently of the valvetrain and timing can be fine-tuned further. the only observable benefit is better fuel mileage because more of the fuel gets burned (higher surface area due to finer spray). but i digress.

[mrdirty]

Originally posted by ygblat
as far as i know, the new BMW v12 is the only engine on the market that utilizes direct injection. the differences are not really that big between it and regular fuel injection. the injector is now in the actual combustion chamber so it allows higher pressure injection and lets the injectors squirt finer mists of fuel. also, the injector now functions independently of the valvetrain and timing can be fine-tuned further. the only observable benefit is better fuel mileage because more of the fuel gets burned (higher surface area due to finer spray). but i digress.

Actually there are a number of benefits to this system:
-Incredible flame front control
-Vertical intake (less intake losses)
-Fuel injection cools charge and can be used to pack in more air if a primary shot is injected on intake
-Very high air/fuel ratio due to increased flame front control and mix control.

Most comapanies are working on this (Nissan, Honda, MB...) but he proprietary technology (namely the injectors and control system) has a way to go before it's feasable on an economy daily driver. The difference is comparable to carbs to EFI, and the tuning possibilities :eek:

Think of it this way:
-Pack in as much air as a turbo will deliver (forget about detonation since there's no fuel in the cylinder to detonate yet)
-Just before the intake valve closes, deliver a shot of fuel which will cool the charge and suck in more air
-Close the intake valve
-As the fuel is being injected, start the flame front ideally zero unburnt fuel is added (as it's being injected just behind front)

all this adds up to potentially big boost numbers and in turn very high Hp. The downside is that you can forget garage tuning a system this sophisticated.

what was this post about again :confused:

[ygblat]

very true. i was just saying that the NA bmw engine doesnt benefit much from it (i think they quoted a 5% hp increase), especially since one of the purposes of this technology was to allow that engine to run lean but the US's wonderful fuel doesnt allow that. the benefits to a forced induction engine, however, would be phenomenal.

back to the original post, heh. as far as i understand, since raising the compression compresses the same amount of oxygen further, more fuel will not be needed. the efficiency goes up, but is expressed in either more hp or less fuel used and hp remaining the same.

[wherezmytofu]

actually, higher compression piston alone will not make you have lower gas milage, higher compression = hp curve higher in the band, normaly need a cam with more duration, higher rpm = more gass shot into cylinders :)