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.

 

[Turbinator]

If I understand your question correctly: The ideal diesel and otto cycle efficiency calculations do not account for heat transfer from the engine to the coolant. This will account for around 40% of the total fuel heating value. Mechanical losses depend on what accessories are attached to the engine.

 

[franzh]

MKimagin
You are falling into the trap assuming that not all of the fuel burns during combustion. Too many websites make that claim, then proceed with their miracle product to make it do so.
What comes out of a tailpipe on a modern IC engine, be it gasoline or diesel, is for all intents and purposes void of any combustible fuel. When you calculate engine efficiency losses there are many, many variables, heat rejection is indeed a fairly large percent. Other losses are of course, friction, thermochemical conversion, and a big one, suction throttling losses on a throttled engine.
If you assume that there are no heat rejection losses, the exhaust manifolds would be ice cold, as would the engine coolant, and the engine would spin over frictionless, forever.
When we do engine development work, we take an engine and motor it on a motoring dyno at various speeds without allowing the engine to run on its own. We can calculate what power is required to just run the engine, long before the engine runs any accessories.
We have a long way to go before we take the IC gasoline engine much over 30% and a diesel engine much over 45%.

Franz

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[drwebb]

"What comes out of a tailpipe on a modern IC engine, be it gasoline or diesel, is for all intents and purposes void of any combustible fuel."

True enough, but how much work is recovered from the catalytic converter?

 

[kenre]

Catalytic convertor doesnt recover anything. It reacts with conbustion products and converts them to less harmful pollutants.

Ken

 

[NickE]

kenre- right, thats what drwebb is saying! IF the catalytic is combusting the final bits of fuel/etc form the exaust, where is the heat it's making being used?

 

[GregLocock]

Some of the unburnt/partially burnt fuel in the feed gas is used to keep the cat hot. Seriously.

Rather more bizarrely, at full throttle many performance engines use a rich mixture to COOL the cat with unburnt fuel.

Incidentally the Prius gasoline engine has a peak efficiency of 37%, and runs above 30% for its entire working range apart from idle. Of course, its working range is limited to one value of torque for a given engine speed.

It does that partly by underfueling.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kenre]

kenre- right, thats what drwebb is saying! IF the catalytic is combusting the final bits of fuel/etc form the exaust, where is the heat it's making being used?

Simple answer to that one! The Heat is used to warm the floor above it, or burn careless hands.

Greg, Sounds like a waste of fuel!

So how does all this compare with making an engine more efficient, to making parts last the warranty or reducing the pollution emmitted? vicious circle....

Ken

 

[MKimagin]

Hi all
franzh - You are right that there are losses in IC engine that can't be avoided due to heat transfer , friction..., however that losses can be consider as constant for particular RPM and load.
We can see in last 10-15 years significant improvements in efficiency and power from similar size engine. I did not notice that the engine coolant temperature and exhaust temperature got significantly lower then in engine 15 years ego. Lowering temperature will suggest mechanical and thermal improvements, is that true? If not , so from where is the improvement?
Well, it look like the timing management, fuel management (that include, when and how well the fuel mixture is madded) and also thermodynamic of the combustion itself. All that play significant role .

franzh - is not a question if all fuel is burns, but when and how fast all fuel is burn - You probably understand that much better then I do.

What I look in the original question is to find how far we can go with particular engine improvement of combustion when we say: that it, that a theoretical limit, now we have to look to reduced the fiction or heat transfer and so on.


GregLocock - you saing that "Prius gasoline engine has a peak efficiency of 37%" - how they got that? That is high.

 

[GregLocock]

Well, attention to detail.

1) Atkinson cycle.

2) Low specific power output (kW/litre)

3) Low redline -> lightweight parts

4) if you only have to optimise for a small range of operating conditions then it is easier to than if you are trying to optimise economy for all throttle openings and all speeds, while optimising for max power and max torque as well.

5) and lots more stuff that I don't know about.







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mikegeo]

"and a big one, suction throttling losses on a throttled engine."

This is the largest reason why cylinder deactivation increases fuel economy?

"We can see in last 10-15 years significant improvements in efficiency and power from similar size engine."

What is your definition of engine efficiency? BSFC? Gas mileage? HP/Liter?

The increase in power (Hp per liter) is not necessarily linked to an increase in efficiency (lets say BSFC). It is largely related to Volumetric Efficiency. It basically comes down to allowing the engine to flow more air, especially at higher engine speeds. Variable valve lift and timing technology has been essential in achieving this on production vehicles. Proper tuning of the exhaust and intake manifolds also plays a large role. Hondas new K20 engine reaches a very high volumetric efficiency because of this, well over 100% (I think it approaches somewhere near 115% at redline, a very high 9,000 rpm).

 

[GregLocock]

If you take a 4 litre engine at 1800 rpm, that is pulling 60 litres per second of air. If it is running at -50 kPa MAP (ie cruising) then it is wasting (crudely) 3 kW in pumping losses across the butterfly. That engine is probably only producing 30 kW in those conditions.

So, if you could deactivate half the cylinders and run at 0 kPa that'd be a 10% boost in efficiency straight away.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MKimagin]

Does it mean that 2 struck engine that do not have intake and out take valves have much lower pumping losses?

 

[SomptingGuy]

No, it means that an unthrottled engine (either one running WOT or one with no throttle - like a diesel) has lower pumping losses.

 

[waross]

Check out an engine manufaturer's site such as Caterpilar.
Look up diesel generators and the specs often list BTU's out the exhaust, BTU's off the radiator, BTU's radiated for the engine block.

 

[tbuelna]

waross,

I used to work with this wise old engineer who referred to it as the "Holy Trinity": one third of the btu's out the exhaust, one third of the btu's to the crank and one third of the btu's lost to cooling and friction.

Obviously, he was a Catholic.

 

[MKimagin]

So not much we can do about internal fiction.
Why we getting 30% loses in to exhaust? That close to the theoretical or just we are not able to properly design the IC engine.
I wonder if we can use some thermo electric effect, on exhaust to recover some energy with out compromise the gas flow.
I so sower on the net thermo-electric generator that use natural gas. The only problem is that it have just 12%. No the greatest number all all, however if you could recover this from exhaust, we could used that for something.
What if we can used instead thermo-eclectic, electro static effect(assuming that there are some free electron in the gas). I could work like capacitor (or heve capacitor) that sequentially getting burst of ionized gas from one cylinder, and push out the electrons from one plate (more like electrically isolated exhaust tube) thru electrical conductor then load (to do the work) to other cylinder exhaust manifold (that act like other side of charge capacitor). Then you will got another burs of charge from other cylinder and reaped the process in reverse.
That look like AC current. Strange, I just thinking ...the system will be open circuit ... and it might work. Electro static is a add thing.

 

[GregLocock]

Ivymike - have you got a breakdown of what the 30% in the exhaust consists of?

Obviously a large part of it is temperature, but it would be interesting to know how much each of the other contributors is.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[drwebb]

"So not much we can do about internal fiction."

Maybe not the 'engineers', but there are platoons of tribologists in industry and academia that are working this problem and making some progress. While 1% efficiency gain may not make much difference to the average end-user, a Toyota executive has claimed that 1% gain in CAFE is worth several billion$ to an OEM. Hence- Big Oil conspiracy theories aside- OEMs are probably leaving few stones unturned . . .

 

[MKimagin]

drwebb
You are right: 1% here, 2% there... and you do know even when it accumulate to 30%-40%, just from the small things.

Can we divided the losses in very small portion, then in deep describe them, and make some brain storm among all interested, and get some ideas how to minimized the inefficiency on each of it.
Some people here have enormous experience in automotive , some in chemistry, some in electrical... , some have no experience in any thing LOL.
Some of the best idea are from people that have no automotive experience at all. Just the fresh look on old problem might be just it.