Power effeciency of an ICE?

[Joe_Oh]

I have been told by individuals that a gasoline powered engine, lets say a 100hp MAX engine, only %15 of that 100hp power actually goes to the wheels and moves the wheels. Is this correct?

If that is correct, you are actually driving a 15hp vehicle instead of the 100hp at the crankshaft. That seems kinda off. Can someone please better explain what I've been told. I dont want to quote bad info.

Thanx.

 

[wingnut]

15% is a number I've heard used often to describe the conversion efficiency of an ICE. In other words, it converts only 15% of the energy stored in the gas into mechanical energy used to drive the wheels. Electric motors on the other had can covert as much as 90% of the energy stored in a battery to mechanical energy.

How much of that mechanical energy is getting to the wheels is really a question of how efficient the transmission is but you won't loose nearly as much there and an electric motor would have the same problem.

 

[kibble]

I've heard similar to what wingnut said. It would make sense seeing how a lot of the energy produced by an engine is converted to heat.

 

[Joe_Oh]

So, when a car is measured at the wheels by a dynamometer, would it be 100hp max with the fictional engine I stated, or would the dynamometer show 15hp? Are the dynamometers pre-adjusted for that?

 

[AutoMX]

it doesn't work like that. no dyno measures the amount of potential energy in fuel, it just measures the force and speed of whatever is moving it. a car has a typical drivetrain loss of maybe 15 to 30%, meaning the power at the wheels is that % lower than the engine's crank is outputting. some dynos have an added feature to guess the power at the crank based on an average drivetrain loss % but it's hardly accurate. they need to take the motor out and test it on its own to find the real #s.

that said, an ICE is not very efficient, but not much stuff is. sure an electric motor is pretty efficient converting electricity to motion but getting the electricity is lossy anyway.
coal, oil, gas, etc.. = major heat loss
nuclear = HUGE heat loss
solar = today only 15% of light goes into electric current
wind = turbulence, drag, and transmission loss = around 50% efficiency or less

and the list goes on. on the upside, it leave people alot of room to improve. imagine if ICE could convert heat to electricity, prolly get 400mpg or better.

 

[ed1380]

what automx said. the 15% is drivetrain loss

 

[Joe_Oh]

so when a car gets 100hp max, that means it gets 100hp "at the wheels?" and may be getting 15-30% more if there were no losses at the drive-train and waste heat produced?

 

[ed1380]

for sales purposes they advertise the crank hp. so remove the 15%

 

[kibble]

 

[carbon]

an engine only manages to use 15% of the energy present in the gasoline...
therefore a 100hp car is getting 100hp of power out of 666hp worth of gasoline
i might be wrong...

 

[wingnut]

HP and energy are two different things. You might say that hp is a measurement of how quickly a motor can convert the available chemical energy (gas) into mechanical energy (motion). Another way to look at it is that you can get a million hp out of a gallon of gas if you can burn it fast enough.

 

[AutoMX]

it's like a battery. if you have 10Ah (amp hours) then you can run 1 amp 10 hours or 10 amps 1 hour. thats the on-paper physics explanation. in reality the majority of the energy in ICE's (as noted) is lost in expelled heat, friction (heat again), and unused motion like the exhaust gases rushing out which turbos use but not to actually drive the motor.

 

[Joe_Oh]

HP and energy are two different things. You might say that hp is a measurement of how quickly a motor can convert the available chemical energy (gas) into mechanical energy (motion). Another way to look at it is that you can get a million hp out of a gallon of gas if you can burn it fast enough.

Yea, that's what we call an explosion!