I just can't see how you could make the intake and exhaust the same temperature, the explosion of petrol in air creates a lot of heat (as well as a huge change in volume - which drives the piston).

The heat is a by product of the chemical reaction.

The only efficient use of it in the car is I think to heat the air for the cabin heater.

The other good thing high temperature exhaust does is get the hell down the exhaust pipe faster, if you let exhaust cool down in the pipe is has more resistance to flow (some call backpressure). That's why there can actually be a case where an exhaust pipe is too big, exhaust slows down and cools in a bigger exhaust.

Hmm am I still on topic? I think so. I have a tendency to ramble...

Mildman,

The theoretical point where you get 100% thermal efficiency is exactly that, theoretical. What the original statement means though is that the lower the temperate in your exhaust (ie closer to the inlet temp), the more efficient the motor will be running. THat is assuming consistent fuel mixtures.

The less heat you are wasting out the exhaust the better as it means more of the energy has been turned into kinetic energy rather than heat.

It's been a while now but I believe that the most efficient a 4 stroke petrol engine can be is about 40%.

As for the turbo part, turbos do make an engine run more efficiently. Like you said they utilise that otherwise wasted heat energy.

As DAveman said, the heat is used to expand the air, which causes the piston to move down the bore. etc etc.

As for hot exhaust temp...the hotter it is the less dense it is..

the exhaust removes a MASS of gas. the reason hotter is better is : because the gas has a higher velocity, it helps draw out all the gas from the engine. It does this by creating a lower pressure region in the exhaust (compared to combustion chamber) so the exhaust gassses want to leave.

Another reason hotter exhaust gasses may be better is because : beacuse the gas is hotter it has more energy thus it is easier to make it leave the engine.

But all of this has nothing to do with thermal efficiency. Thermal eff is a part of engine design but, in the simple context I understand it in, it's not the holly grail of engine power, it's more likely, that it is closely related to fuel efficiency.

Hotter is probably better, I would like to know up to what point though???

I understand that the intake and exhaust temp should be the same, meaning all the heat became some kind of work.

Second part of the question. Cold air intake? some people think slightly warmer air is better, even cold air gets warmed up - what's the density change really?

can i just tell you an intersting figure.

only around 25% of the energy produced is actually used to power the car forward.

the remaining 75% is given off as heat and noise.

to increase the effieciencey of a car then you obvoiusly need to reduce the amount of wasted energy and this requires (insert the name of device that would be worth millions if some1 were to invent it) something to use the heat energy and convert it to kinetic energy.

reducing the exhaust temp by using water or something to cool it wont increase the engines effieciency. it needs to be cool to start with, but as combustion takes place in a combustion engine (imagine that ay?) its hot to begin with.

Quote:

I don't think anybody was implying this.

I'd say some of you have but for others i'd suggest reading up about the carnot cycle. It's basically an ideal heat pump (car engine) and you can use this theory to work out the maximum efficiency for any engine. What it basically says is that the maximum efficiency for a heat pump is (T(high) - T(low)) / T(high). where T high is the temp at which your fuel burns at and T low is your low temp reservior (ambient). For this you need to work in degrees Kelvin (add 273).

For instance lets say it's 20 degrees ambient and your fuel burns at 1000 degrees. That makes

(1273 - 293)/1273 = 77%

Already we have lost 23% just through the design of our entropic universe!!

Damn our universe! How am i ever going to get 200HP/Liter?

so the universe is robbing us of 80% of our power...

still looking for a chart of air density vs/ temp...