There is more to it than that. Compression that can occur without increasing the temperature of the fluid that is being compressed will require less work. So the closer we can come to this theoretical situation the less energy will be consumed during compression.



It also reduces the friction loss as the indicated cylinder pressure decrease with temperature.

The suggestion was cooling after the compressor and that will only save compressor work in line with the reduced boost pressure. Volume flow through the compressor will be the same. Compressor work is of course the integral of VdP.

You are correct about isothermal compression requiring less work than isentropic but that requires cooling during the compression process - not after.

The roots isn't a compressor, it's merely a pump where compression occur at the pump outlet which can be cooled.

Of course, completly isothermal compression isn't realistic.

The aircraft and GE staionary turbine data makes me ask three questions

1) the aero jet obviously gives less power at high altitude but the plane has less drag. As I understand it a plane can stall at high speeds ( 500mph+) at high altitude due to thin air and this may be why planes "weave" in mid atlantic etc to get air traffic spacing right.I.e if they just throttled back they would stall. So is the ratio of drag versus power a constant or not

2) What sort of efficiency does a fanjet get at high altitude , GE quote 50% for a turbine generator set at sea level, does a big fanjet get over 50%?

3) GE quotes 80% efficiency with exhaust heat extraction. If I give up ,say 20% in transmission losses and battery cycling does this make an electric car fundentally more efficient than a dieasl engine on the road even if the turbine generator uses fossil fule.

My simple calculation - 80% * 0.8 = 64% versus about 40 to 50% for a car diesel.

Your numbers are way out. Small turbines are inefficient. Recapturing heat energy from the exhaust is not economically viable in automotive applications, and if GE just add the heat recovered to the shaft power they are bordering on fraud in my opinion, due to the nasty old second law of thermodynamics.

I don't think the OP said it's a Roots. Compression in a Roots occurs when high pressure air flows back into the chamber.

I don't see an aftercooler reducing the pumping work in any compressor or blower except in proportion to the reduction in boost.

At constant aircraft speed thrust=drag


50% is too high. The best gas turbines achieve 40% although a large turbo-fan might be a little higher if you include the thrust delivered by the exhaust.

Combined cycle turbines achieve up to 60% TE by using some of the waste heat in the exhaust to drive a steam cycle.

Large diesels are achieving over 50% (much higher than the best gas turbines) and this will probably increase as manufacturers explore the limits of compounding (exhaust energy recovery).

Actual wording from the GE brochure

Attaining more than 80% utilization of the fuel input by a combination of electrical power generation and process heat is not unusual.

No, you can slow down a fair bit in-flight in an airliner. For example in a 747 you would normally cruise at about M 0.85, but if need be you can knock off about 10% of that no probs. Though it's easier to do that when you're light rather than heavy as the low speed buffet margin is higher. The only reason they'd 'weave' is to find better winds, not to lose speed.
I'm not sure of the efficiency in the cruise but I think it's up around 50%, or about the same as the best diesels.

GE Gas Turbine paper

Hidden in table 1, you will find the thermal efficiency. Column 5 - Heat Rate (kJ/kW.hr) is fuel heat energy input per unit of shaft energy output - a form of BSFC. Since one kW.hr = 3600 kJ just divide 3600 by the number in column 5 to get the thermal efficiency. The best efficiency for those in the table is 0.37 (37%) for the PG7251 (FB) running on gas.

Figures 9 and 10 show the LOSS of power and efficiency associated with increase in inlet temperature and increase in altitude.

The most efficient commercial diesel I know of is around 54%, not surprisingly it has a turbo.

Yes I don't think that efficiency could be achieved from a NA diesel.

Most efficient NA is ~50%. There's some good reason why diesels get more efficient with a turbo, whereas gasoline engines don't, as a trend. But I can't remember the story.

If you eventually remember, Greg, I'd like to know why - I'm intrigued.

I'm kinda hoping TDIMeister or JEdlund will chime in, I'd like to know again.

I can think of three to start:
1. Operate at higher excess air values
2. Higher bmep means proportionally lower friction losses (fmep)
3. For four stroke diesels, an efficient turbo set-up with boost > exhaust back-pressure sees pumping work go negative - ie waste heat is being converted to useful work at the crankshaft.

Greater CR.

Greater ER.

No, a diesel's efficiency is determined by its compression and cutoff ratios. If you examine its p-v diagram... oh never mind. On second thought, just cut off your head and become a lighter, brighter person.

Isn't it because the combustion temperatures hence exhaust get higher under load in a diesel and able to drive the turbo harder but light load the turbo supplies little boost or something like that?

Meh, diesels, whats the fun in an engine that doesn't 'wind out'.

It's all very well to examine it - and parrot the Thermodynamics textbook - but if you don't understand what you're looking at . . .

If you examine the Diesel PV diagram and use air standard cycle analysis you will indeed find that the efficiency is a function of CR, Cutoff ratio and the ratio of specific heats of the working fluid. You will also see that CR is equal to ER. For cycles where ER is different to CR (eg Miller) the thermal efficiency depends on ER not CR.

Back to Diesel PV. You will also note that CR used for TE calculation does not include the external compression provided by a supercharger. Increasing CR via supercharging does not increase the thermal efficiency in fact it will reduce. Even using turbocharging with CR=ER does not increase thermal efficiency. It is only when boost pressure > exhaust back pressure that any of the recovered energy can be returned to the crankshaft (during the pumping loop) and this only applies to four stroke engines.

In short - turbocharging does NOT increase TE by increasing CR.

Hi limited slip dif..,

Well,A very important advantage of fuel injection is that it eliminates the risk of carburettor icing, as there is no carburettor! According to the General Aviation Safety Council in the UK, carburettor icing causes on a yearly average 6 to 10 accidents with serious injuries or fatalities.Carburettor ice is most likely to occur when temperatures are below 20°C (70°F) and the relative humidity is above 80 percent. However, due to the sudden cooling that takes place in the carburettor, icing can occur even with temperatures as high as 38°C (100°F) and humidity as low as 50 percent. This temperature drop can be as much as 20°C (70°F)!As a preventive measure against carburettor icing, one can apply carburettor heat. It is an anti-icing system that preheats the incoming air before it reaches the carburettor. Carburettor heat is intended to keep the fuel/air mixture well above its freezing temperature to prevent the formation of carburettor ice.However, there is a disadvantage to its use: the use of carburettor heat causes a decrease in engine power (sometimes up to 15 percent!), because the heated air is less dense than the outside air that had been entering the engine. This also enriches the mixture as the carburettor does not compensate for air temperature.

Thanks

Never said it did. That's stupid. You are only trying to launch another of your endless, stupid arguments. Sorry, not interested.

Sorry McGuire - I misinterpreted the following post - it sounds like your are saying "Greater CR" is "some good reason why diesels get more efficient with a turbo". What exactly did you mean?

The main thing working against gasoline engines when a turbo is added, is that AFR needs to be richer to combat detonation, piston/valve temps etc. Of course any fuel added in excess of stoichiometric cannot be burned and must come straight off the TE bottom line. Apart from this, I am sure the TE of a gasoline engine could be improved by turbocharging as with the diesel.

Diesels on the other hand, always run in an excess air condition and this can and will be more so when turbocharged.

Another factor is size. Diesel efficiency continues to increase with increasing cylinder size whereas gasoline engines peak at a comparatively small cylinder size. Consequently diesels tend to populate the larger end of the market. Turbochargers also increase in efficiency with increasing size so fitting a (large) turbo to a large engine is more likely to result in an improvement in TE than a small turbo added to a small engine.

Not as stupid as you might think. CR can be considered to include external compression from a supercharger so CR(total) = PR(sc) x PR(cylinder).

Fact is - although we use CR in the efficiency formula for Diesel, Otto etc cycle, TE is more closely related to ER which just happens to be the same value as CR in most engine cycles. So when a turbocharger is added, the increase in overall CR does nothing for TE. On the other hand, an incease in overall ER can benefit TE if the recovered energy can be applied to the crankshaft. As mentioned previously, this can be achieved in turbo four strokes if Boost>Exh back-pressure. It can also be achieved by mechanical or electrical connection to the drivetrain as in turbo-compounding or with a piston expander as in the Ilmor 5-stroke.

"My" endless stupid arguments have invariably been launched by yourself (see below) and I suspect they only rate as "stupid" because you have been proved wrong on every occasion.






(I THINK THIS IS THE POINT WHERE SOMEONE IS TRYING TO START AN ARGUMENT!!! TYPICALLY PLAYING THE MAN NOT THE BALL I MIGHT ADD.)



SORRY MCGUIRE - THAT'S EXACTLY WHAT YOU SAID. IF YOU MISINTERPRETED SOMETHING, PRESSED THE WRONG KEY, MEANT TO SAY SOMETHING ELSE - JUST SAY SO. DON'T TRY TO BLAME SOMEONE ELSE. PRETENDING TO BE PERFECT IS THE WORST IMPERFECTION OF ALL.

80% include 20% heat, so the efficiency to electricity is 60%. Transmission losses are close to 10% and an electric vehicle is probably around 60% efficient. But many powerplants also operate with an efficiency not much higher than 30%. On the other hand, a passenger car engine operate with an average efficiency much lower than its peak efficiency.

To compare fanjets with turboshafts on the ground can be quite difficult.



He said GM style which I assume means a roots type pump.

Some leakage will always occur, but at lower pressure ratios and higher speeds the leakage shouldn't be that high.



The problem with the gasoline engine is that the compression ratio in general must be decreased when turbocharging, otherwise knocking would be an issue. But with a fixed compression ratio and ideal fuel and ignition timing the efficiency of a turbocharged gasoline engine should increase with boost pressure as long the exhaust pressure is kept down. With a diesel on the other hand there is no need to reduce the compression ratio to avoid knocking when turbocharging the engine. There is however a cylinder pressure limit but that is nothing that pose a significant problem.

When the engine is turbocharged imep increase more than fmep and as a result the engine mechanical efficiency increase.

With turbocharged gasoline engines the focus tend to be on improving the efficiency at part load rather than to increase the full load efficiency. Even if turbocharging decrease the peak efficiency it can significantly increase the average efficiency of a passenger car gasoline engine. Such an engine run with an average bmep of only a few bar, too little to be efficient.

60% would be a combined cycle plant with Rankine (steam) cycle utilising waste heat in the exhaust. Approx 40% is max TE for large GT's.

I wasn't talking about leakage - I was referring to the backflow from the pressure side back into the blower as the rotor uncovers the discharge port, exposing the low pressure contents of a chamber to the high pressure in the engine intake.

The apparent improvement in efficiency via turbocharging in diesel engines, not evident in SI gasoline engines, is due to the fact that turbo diesels can run considerably greater compression ratios. In the classic idealized p-v depiction, the diesel cycle is constant pressure while the Otto cycle is constant volume (approximations). So in principle, the Otto cycle is actually more efficient than the diesel cycle, all things being equal. But they are not equal since the Otto/SI engine is knock-limited. The diesel can run a considerably greater compression ratio.

A normally-aspirated SI engine with its compression ratio fully optimized for maximum engine efficiency runs on the outer limit or threshold of knock. From that optimal CR and efficiency, additional cylinder pressure (via turbocharging, for example) will not increase efficiency but only push the engine into knock. Consequently, with any significant increase in manifold pressure the engine's compression ratio must be reduced, thus actually reducing the engine's efficiency vs. normally aspirated. The turbo diesel's compression ratio is not constrained in this manner.

That’s not compression ratio. You are actually referring to a related property known as “equivalent compression ratio,” not to be confused with true compression ratio or other related terms, including effective and dynamic compression ratio.

Some terms you’ll want to know:

Compression Ratio: the ratio of cylinder volume at BDC vs its volume at TDC. CR is a statement of volume ratio independent of pressure, for example: If an engine has a compression ratio of 10:1, its compression ratio remains 10:1 at sea level or at 30,000 feet; normally aspirated or at 40 psi boost.

Effective Compression Ratio: Also known as Dynamic Compression Ratio. The ratio of cylinder volume at IVC (intake valve closing) vs cylinder volume at TDC. Especially relevant in boosted and Miller cycle applications.

Equivalent Compression Ratio: Compression ratio times manifold pressure in atmospheres. Somewhat misnamed, as its value does not reflect an equivalent true compression ratio. Used mainly by the aftermarket turbocharger/supercharger industry as a back-of-the-envelope rule of thumb in identifying the approximate knock limit in boosted engine conversions. Of limited value in serious engine development.

That is a combined cycle efficiency yes. A large open cycle gas turbine without recuperator reach efficiencies in the 40-45% range.



The diesel engine actually operates using the Seiliger cycle which is heat added during both constant volume and constant pressure. But at high load the real diesel cycle is quite close to the real otto cycle. In reality I also wouldn't expect heat addition during constant volume to be that efficient, the heat losses would more than compensate for the increase in efficiency offered by the ideal cycle. This have been noticed in HCCI engines.

With SI engines the type of fuel and the knock resistance of the engine design will have a significant effect on the efficiency when turbocharging. There comes a point where a naturally aspiranted engine will no longer see an increase in efficiency when the compression ratio is increased, and the efficiency increase offered by increased compression decrease the higher the compression ratio becomes. I would expect to see maximum efficiency with a compression ratio in the 14:1-18:1 range.



The only relevant value here is the compression ratio, both equivalent compression ratio and effective compression ratio is of limited value. With the effective compression ratio there are dynamic charging effects which will change the real situation quite a lot from the situation seen on paper when calculating using IVC.

In terms of thermal efficiency it's the expansion ratio that is the real key, but as ER often equals CR this is often simplified to mention only the compression ratio. If we use an external compressor this will in effect increase the compression ratio using a two stage compression, but as it doesn't increase the expansion ratio it will not increase the thermal efficiency. I think this is what gruntgury is trying to point out.

At max efficiency the SI engine's CR is knock-limited, so the addition of more pressure via turbocharging at that point will not increase efficiency but only push the engine past the knock limit. The diesel does not suffer this limitation in compression ratio. That's pretty much the long and the short of it.

NOTE: Compression ratio, not expansion ratio. ER and CR are typically symmetrical, but expansion ratio itself has no direct bearing on knock. So while ER is certainly relevant to engine efficiency in general, it is not relevant to the question posed here.

Like the Otto and diesel cycles, the Sabathe/Seilinger/ et al cycle is also an idealized depiction. It may be more useful or accurate for a number of purposes, but not this one.

Compression ratio is also subject to dynamic filling and backing, just like effective/dynamic compression ratio or equivalent compression ratio. The difference for me is that I find CR and effective CR to be useful values in engines; equivalent CR, not so much. If you want to know why one engine combination knocks while another, seemingly similar combination with identical CR, chamber configuration, etc, does not, have a look at effective CR. Especially useful on octane-limited combinations. However, over time I have seen that in practice, equivalent CR doesn't really track anything in particular except in the broadest way. It's a calculation of apples (compression ratio) times oranges (manifold pressure). Not being equivalent properties, they don't really have equivalent values.

Semantics. You know what I meant.

Ahh now we understand what you meant.

The CR story is one of the reasons diesels respond better to turbocharging than SI. However it doesn't answer the main part of the original question which was "There's some good reason why diesels get more efficient with a turbo".

Here are 3 good reasons.
1. Operate at higher excess air values (when turbocharged)
2. Higher bmep means proportionally lower friction losses (fmep)
3. For four stroke diesels, an efficient turbo set-up with boost > exhaust back-pressure sees pumping work go negative - ie waste heat is being converted to useful work at the crankshaft.

You only need to compare the formulae for efficiency of the ideal diesel and otto cycles to confirm that. For any given compression ratio the otto cycle (heat addition at constant volume) is more efficient than the diesel cycle (heat addition at constant pressure). This is due to the heat addition occuring at a higher average temperature of the working fluid, for the constant volume case. As you pointed out, the Seiliger cycle approaches otto cycle efficiency as cutoff ratio approaches zero.


I would expect HCCI to approach the ideal otto cycle (heat addition at constant volume) more closely than SI due to the shorter combustion event.


Is this because the increase in heat loss due to the increasing peak combustion temperature becomes greater than the increase in TE?

There is actually a lot more to it, as I said a couple of posts up.

The phenomenon you mention is an efficiency advantage held by all Diesels - both turbo and NA. The NA Diesel cycle has lower
TE than an Otto of the same CR, but the diesel overcomes this with its ability to operate at much higher CR.

In the real world, the diesel has three other TE advantages
1. The ability to operate with excess air at full load
2. The ability to operate without throttling (and the resultant pumping-work losses) at part load
3. The ability to concentrate the combustion process in the central regions of the chamber - thus reducing flame quenching and heat loss to the walls. (SI engines with stratified charge and/or direct injection may achieve the same goal)

You might not have noticed - but the question posed here was about efficiency - not CR.

Although the efficiency formulae always refer to CR, this is purely coincidental and only applies when CR=ER. It is in fact ER that has the greater bearing on efficiency.

Knock limits the SI engine's CR, not its ER. The diesel's CR is not constrained in that fashion. Simple as that. You are just being argumentative.

Correct. Correct. Incorrect. Incorrect.