What are the real efficiency losses in IC?

[MKimagin]

I wonder what is the efficiency loss for diesel and Otto engine in terms of mechanics and inability to cum bust fast enough the fuel.
I can calculate the absolute thermal efficiency, let say diesel engine with with compression 45 to be around 78%. The real efficiency (I read somewhere) is around 45%. The difference is 33%. Now from that 33%, what part is the mechanical (friction) loss for four struck engine, and what account for inefficiency in fuel combustion (not completely burn fuel particle CH,CO...) and what account to pre ignition (knock) due to necessary timing setting.
Which engine (Diesel or Otto) have higher losses (stated above) compare to there theoretical efficiency.
I understand that this is closely connected to RPM.

 

[crysta1c1ear]

GregLocock (Automotive) 25 Jan 06 6:17
Ivymike - have you got a breakdown of what the 30% in the exhaust consists of?

Am I missing something Greg? Isn't it practically all temperature. There is obviously some kinetic energy resulting from flow past the exhaust valves.
Have you anything else in mind apart from temperature and (easy to calculate) kinetic energy?

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According to

http://sitemaker.umich.edu/mhross/files/fueleff_physicsautossanders.pdf

PDF page 17 of 29
the breakdown is like this (all percentages being from the original 100%, not percentages of what is left) for a US composite driving cycle

100% in fuel
30% irreversibility of combustion,
puts 70% into combustion gases

32% engine inefficiencies, (into cooling system and exhaust gases) and
18% friction (including pumping losses, driving camshafts etc)
leaves mechanical energy 20%

2% to drive accessories (oil and water pumps, air con, etc) and
3% lost in torque convertor transmission etc
leaves 15% to the wheels

5% used for heating the brakes
5% for air resistance
5% for rolling resistance

The author comments in the text, that this is for a particular vehicle and the driving conditions mentioned. Most drivers would drive faster today and have more air resistance relative to rolling resistance.

 

[waross]

Here's some specs for a big Cat. Not to contradict but to compare and comment.

Caterpilar 700 kW generator
Cat diesel Engine
3412 TA V-12 4-stroke-cycle watercooled diesel.
Fuel consumption at full load 51.5 Gal/hr
No 2 diesel fuel with 35 Deg API and lhv of 18390 btu/lb.


Heat rejection to coolant (total) 25478 Btu/min
Heat rejection to exhaust (total) 42766 Btu/min
Heat rejection to atmosphere from engine 6028 Btu/min
Go to

http://www.cat.com/cda/components/fullArticle/?m=39280&x=7&id=215813&languageId=7

and download a spec. sheet. The smaller sizes are in a different format.

 

[jons999]

Back to the cat convertor thing for a second. Many cars use something called Fuel Cut Tau which actually pulses the fuel injectors occasionally and pumps raw fuel out the exhaust during deceleration when the engine isn't firing. This fuel is used to keep the cat up to operating temperature during long periods of closed throttle decel.

-Jon

 

[Rob45]

One way exhaust heat energy is recovered, of course, is with a turbocharger, that is driven by exhaust enthalpy, i.e., both temperature and pressure.

The amount of fuel used in keeping a cat operating is very very small, in the overall scheme of things.

 

[shanba]

BMW has announced a system to recover some of this exhaust energy and do work directly on the crankshaft. They use steam as a energy transfer media.

http://www.greencarcongress.com/2005/12/bmw_developing_.html

 

[MKimagin]

shanba , very interesting.
People what you thing about that idea?
I wonder if just using one turbine with electrical generator (not full turbo charge) on exhaust could recover some of the exhaust losses. The electrical generator will provide power for electrical motor that could be connected to crank shaft.

Even if the turbine will be left free running it could recover some power. The turbine will get access pressure from cylinder when the piston is in the lowest position, then when the piston start moving up the kinetic energy stored in the spinning rotor pull the exhaust and the turbine should act like vacuum pump, reducing the pumping losses.
Probably the rotor have to very light