O/T Diesel Engines

[Alias]

I recently drove an Audi A3 TDI. And the following questions arose. Can anyone enlighten me???

 

a) why can't diesels take revs over 4500rpm??

b) why does all the torque disappear over 4100rpm

c) I know it has a turbo, but how can a 1.9 diesel car get 220lb/ft of torque and a petrol equivalent can't?

 

All answers to further my knowledge gratefully received.

 

 

 

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1400 (non-starting) Supersport with 6 gears and clamshell wings ☹️.

[Roger King]

I'm happy to be corrected by a diesel expert on this, but here's my twopence worth -

 

1) The speed of combustion is much slower in a diesel for various reasons - the fuel burns more slowly, it is squirted into the combustion chamber and this takes time, etc. Therefore, big revs don't happen.

 

2) See 1) above

 

3) The main reason for the large torque output is the turbo. Because a diesel can't rev, the power has to come from huge torque outputs at lower revs, and therefore the tuning and set up is biased towards cramming as much air in as possible at low revs. Non-turbo diesels don't have particularly huge torque figs.

[Peter T]

Here goes... One benefit is that Diesel engines are not knock sensitive unlike their petrol counterparts so substantial fuel imprvements are to be gained, power output on a diesel is dependant on the weight of the fuel burned and the engine speed is dependant on the amount of air it can inhale.

So turbocharging a diesel increases the amount of air it can inhale therefore increasing the amount of fuel which in turn produces more power. The limiting factor on a diesel engine revving is the mechanical and thermal capabilities of its structure.

Any commpression ignition engine or petrol variant will certainly have a higher torque figure than an engine without a forced induction.

Remember that diesels have a very high compression ratio of 18+1, so all components have to be immensely strong to cope with the combustion process.

 

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[Tony Martyr]

A Deisel (Compression ignition) and a petrol (Spark ignition) engine are both heat engines and both work under the same laws of thermodynamics. The power from both is determined by the fuel mass of fuel ingested per cycle (more fuel per ignition = more power). There are some inherent differences. The high compression ratio required to raise the temperature of the aircharge to the point at which fuel oil will ignite requires the CI engine to be of heavier construction but the thermal efficiency is inherently higher. The CI combustion process is slower but the force on the piston created (which translates as torque at the crank) is higher. The greater the fuel charge from a turbo the greater the torque for any given speed. At the same rotational speed the SI engine is unable to extract the same energy from the same cylinder volume

Most automotive CI engines have been optimised to produce peak torque at sub 2000 rpm because of the jobs required of them but there is no reason that CI engines can't be produced that match typical V8 SI engines having peak torque around 5000 rpm and peak power at 5600 rpm. There are CI engine under development to compete at Le mans where with the correct gearing they will have a significant fuel consumption advantage over SI units.

 

[Tony Martyr]

Besides forgetting how to spell diesel !

I forgot to add that diesel oil has a higher energy density per volume than petrol, a fact that is often forgotten and runs counter to popular advertisment themes.

[Peter T]

Oh and don't forget that it is a b*gger to get off your shoes.

 

Support the 7 Society...... Ehhh?

[Joe 90]

The impressive torque figures on turbo diesels reflect the fact that they can run a lot of turbo boost. The bottom end doesn't need strengthening and the reduced compression ratio makes life easier in a number of areas.

 

SEP field working, not spotted in 102,200 miles. Some photos on webshots, updated 10 June

[charlie_pank]

Or you bike tyre :(

[Super_Rich_Bernie]

When I first drove a modern turbo diesel I was surprised at how little pull there was from really low revs. My wife had a Golf TDi on loan for 3 months, then an A3 TDi. In both cases accelerating with reasonable vigour from 30mph requires a drop into 2nd gear and about 2200rpm. I expected them to pull from very low engine speeds.

 

And then ofcourse it is all over by 4000rpm!

 

Jonathan

[Gee Whizz]

yes the new Audi A3 20ltdi is a bit like that

 

nothing everything then nothing again. makes the 6 speed gearbox a must to keep the rev's right

[Tony Martyr]

Turbo charging a diesel increases power by increasing the work done per engine cycle and this increases the b.m.e.p.

It has proved more sensible to increase power outouts via charging development and combustion chamber design rather than by increasing power via increasing rotational speed, this is due to the rapid rise of mechanical and aerodynamic losses with increasing rotational speed and therefore a fall in brake thermal efficiency.

Typical fan designs (not my subject!) seem to have performance curves with a pronouced optimum performance, which tends towards a (4 stroke) diesel engine that has what could be called a pronounced 'sweet spot'. This is what a number of contributors have noticed. It is also the reason that large trucks use hydostatic wheel drives powered by a highly efficient tubocharged deisels that run under govenor control at optimum performance. Any owner of a turbo charged diesel car will discover the gear and speeds that can return impressive fuel consumption running at this point

[Rust Red Seven]

I stand to be corrected but the following is applicable for the power derived from an air standard cycle engine.

 

Thermodynamics basically falls the two standard air cycle engines into the Otto cycle (Speark ignition) and the Diesel cycle (Compression ignition engines) there is a third the sterling cycle but this air engine does not tranfer air across the boundaries of the cylinder and approchaes carnot efficiency but naff all torque, but very silent as there is no bang bit.

 

They all follow the basic suck, squeeze, bang, blow approach even gas turbines and two strokes.

 

Now the theory bit.

 

The power developed from the particular cycle is proportional to the area under the PV diagram and as the diesel has a very much higher high P it does a lot more work per stroke for the same capacity engine. hence for same capacity engines normally aspirated the diesel does does produce a lot mpre power on a stroke per stroke basis.

 

Problem comes with the moving masses and as stated earlier the petrol engine is a lot lighter in construction to overcome the lower compression forces than the diesel and the diesel has to put a lot more stored energy from the flywheeel back to the piston to compress the fresh charge of air to ignition temp of the diesel.

 

Turbo charging does two things now to get to the maths

 

Power = MxBMEPxV

 

M- mass flow across the cylinder and is a combination of fuel flow and mass of air.

BMEP= Brake mean effictive pressure is the average pressure on the piston for calculation purposes

V = swept volume of the cylinder

 

So if you compress the air into the cylinder you get 2 effects and increase in BMEP as the compression goes up and an increase in the mass flow across the cylinder as the air is more dense for the same swept volume. Compressing air makes it hot and the hotter it gets the less dense it is thus the need to cool the air in high compression thus intercoolers.

 

Thus supercharging and turbocharging either type of cycle has benificial effect. Obviously turbo charging is better as it uses the waste energy in the exhaust gasses to compress the air, but you have to get those exhaust gasses going before you can compress hence the turbo lag.

 

Back to the second law of thermodynamics the heat has to flow from a high temp source to a low temp sink and the machine that extracts work in between is a heat engine be it a steam turbine gas turbine or good old pistons and carburetters. the higher you can run the top temp and the lower you can run the bottom temp the more power you can extract.

 

So all this gas flowing etc gets us more power buy allowing the mass flow of air in and out easier absobing less pumping energy in the engine giving more shaft power.

 

If you realy want to make them go try raising the temp of the engine. There are two physical constraints to this. the heat sink is stuck with ambient air temperatures say 25 celcius the hot end is limited by the melting point of the metal we make the components from. Hence if you were to use ceramic bores and pistons and heads you could raise the temps of the top end of combustion and make it a lot more powerfull. PROBLEM is the mechanical strength of high temp resisting material tend to be brittle not good in tension and there is a lot of this in reciprocating pistons.

 

Ford did run tests late 80's on ceramic engines but could not get the reliability, also lean burn technology for emissions control but this was scuttled by the governments adoption of cat technology

 

Lean burn also helps raise the temp as you get complete combustion running on the stokiametic mix but the hot combustion molecules tend to eat at the materials, eg pitts heads and piston crowns also detonation problems.

 

Have you guys noticed that on cold days the engine seems to have more umph when warmed up, like wise on very wet days there seems to be more horses at the rear wheels. First is due to the heat sink being at a lower temperature and the second is the moist air has a grater mass thus increasing the mass flow.

 

Back to the original question a diesel does produce more power per stroke than a petrol engine. A petrol engine can rev a lot faster than the diesel because of the less losses due to compression and the rotating masses. Power is also torque x rotational speed thus high torque low revs is same as low torque high revs hence a petrol does produce a similar power at the shaft but not as thermodynamic work under the pistion.

 

Practical example of this is the generator engines i work with. V16 caterpillar 72,000 CC at 1000 rpm turbo charged Diesel 1400kw (1876 HP)Petrol 1000kw (1340HP) so comparing like for like on a fixed swept volume basis the diesel is more powerfull.

 

Witter over hope you found it interesting

 

As an aside the biggest machine i have worked with are the GE frame 9F gas turbines which are 325 Mw (441000 HP) that would make a seven go 😬 😬 😬. If you want a comparision the Rolls Royce trent found on the airbus is 50Mw (67024 HP) so they are pretty big beasts

 

Rust Red

[Tony Martyr]

Rust Red (good handle ;-)

All OK except para concerning moist air giving more power. Common but false interpretation of observed effect. When air containing water molecules is ingested into the cylinder the H2O replaces air molecules and reduces the mass fuel value. The gain from water injection is entirely a net gain from absorbtion of latent heat of evaporation that has reduced the temperature of the air charge. Cold is good wet is bad and moist is OK providing it is sufficiently colder than dry to overcome the lack of O2 due to water.

I have stood on the crown of deisel engine pistons while the baring motor was running to inspect the bores of a 9 cylinder marine engine with a top speed of 108 rpm. Had to throw my shoes and overalls into the Tagus river afterwards - it was before enviromental policy was invented

[Nifty]

Tony,

 

Don't the big trucks (i.e. those used for quarrying, open cast mining etc) use diesel-electric, like trains, rather than hydrostatic as you previously suggested?

 

Hydrostatic motors run by governed diesels are used a lot in agricultural machinery but not for road vehicles, why is that?

 

Keep off the straight and narrow 😬

[Graham Perry]

Nifty, I don't think hydrostaic is that efficient so it is not used on road trucks, I suspect Tony will know. Diesel electric may not be far away for road trucks. I know hybrids have been tried where the electric motor becomes a retarder for braking.

 

The mining trucks are divided into mechanical and diesel electric. Diesel electric (AC) is being led by Liebherr and Hitachi Euclid. The ever conservative Caterpilar are still mechanical but if rumours are correct not for much longer. Diesel electric has many advantages in terms of simplicity compared to expensive & fragile mechanical gearboxes that have to handle enormous torque and shock loadings. It offers increased gradeabilty and will work better in future driverless applications. When the next generation of 400 ton payload truck are released soon, they will all be diesel electric.

[Nifty]

Cor blimey...400 tonnes *tongue*

 

I've seen a 300 tonne affair out in Aussie, open cast mining.

Had a fire escape ladder on the front to access the cab..several stories off the floor *thumbup*

 

Keep off the straight and narrow 😬

[Nifty]

I guess we use hydraulic because of the extensive use of rams in these bits of machinery and the requirement for infinately adjustable speed

 

Keep off the straight and narrow 😬

[kjp]

I think we have all lost the plot a bit.

 

The output from a single cycle in SI or CI engine is limited by the amount of fuel you burn.

 

In an SI engine you pretty much have to burn the fuel in a fixed ratio to the air (OK slightly richer that stoichiometric for peak power but lets not split hairs).

 

In a CI engine you are not constrained by the AFR thing (in the fuel lean dirction anyway), but there are other limits. First of all you run into a smoke limit and eventually a thermal problem. The truth is however, you run into a smoke limit long long before you reach a stoichiometric mixture therfore for a given amount of air the amount of output from a single cyle in a CI engine is less than possiblr in an SI engine!!

 

So the reason Diesel engines make more torque is because they flow a lot of air (ie: they are turbocharged). If you look at a typical 2.0l Diesel engine is does about 20bar BMEP which is very similar to a 2.0l TC gasoline engine.

 

I accept the argument about thermal efficiency and compression ratio but this is quite a small effect once you get to the high ratio's associated with modern engines, SI and CI.

 

The reason we see better fuel consumption from the Diesel engine is it's part load unthrottled operation, but thats another story!

 

Ken Pendlebury

 

[Rust Red Seven]

Tony concur with your counter argument.

 

But it does have a signifigant effect in very small quantities if subject to the compression process too much drops out under compression and tends to emulsify the sump oil as it passes the rings

 

I did direct water injection research into high speed diesels on a Ricardo E6 research engine in Kingston back in the early 80's and on a 500cc single at 24mm3 per stroke we go a 10% increase in power and a 30% drop in fuel consumption. As you say a three fold effect. Turning water droplets to steam at high pressures and temperatures soaks up a lot of heat from the combustion process. This would normally go out through the cylinder walls and be dumped to the colling system. Secondly steam expands more efficiently on the piston than just the straight fuel /air / combustion gas mix. Thirdly you are stuffing more mass flow across the system boundary per stroke. Compressiong water is also very efficient compared to air so the pumping losses are not great getting the water up to injection pressure. the injection pump was driven off the camshaft.

 

One bit that was exciting was the injection timing. Too early and it quenched the hot air and it could not establish the flame front from the swirl chamber and too late slammed the con rod into the crank as it got a big boost half way down the stroke. about 2 to 3 degrees after the diesel went in was good. Likewise too much water put it out and not enough the power went down proportionally.

 

Fun Eh

 

It has to go in on the top of the stroke though. A tidgy bit in the induction path does increase it slightly. but as you say too much Bu$$ers it up

 

One other bonus was the engine was permanantly under de coke conditions clean as a wistle after 500 hours running.

PS where these Merlees Blackstone marine Diesels (I have deen inside a 20 ft diamiter pot 3000 bhp per cylinder)

Rust Red

 

Edited by - Rust Red Seven on 26 Aug 2004 13:40:52

[krum]

I find this all really fascinating. I do have a question regarding turbocharging, and if it's a daft question I apologise, I know little about IC engine theory.

 

If the primary disadvantage of turbocharging is the need to wait for enough exhaust gas flow to spin the turbo up to speed, why haven't manufacturers:

 

a) Used some sort of semi mechanical drive system that only uses this relatively inefficient method until the exhaust gases take over, or

 

b) Store some compressed gas (the wastegate blows out waste air at the end of a charge cycle, right? If not, must be other places you could find to get waste air from, like all the stuff flowing past the car whenever you move) to give the turbo a helping hand on startup. Is this how the anti-lag systems work on Rally Cars? Or

 

c) Use the waste energy from the exhaust gases to drive a generator to charge a battery then use the stored energy to drive an electric impellor to compress the inlet charge?

 

Love to know if any of this has been done before. If any of this works, just send me the royalty cheques when Ford start using it

[Sheds Moderator]

Krum, you have just invented the supercharger.

Oh, somebody got there first...b**r.

A SC is a mechanical air pump that overcomes some of the lag issues of which you speak and boosts performance. I think the drawbacks are mechanical complexity and cost. A turbo after all is pretty cheap and simple.

[robbieclark]

I'm sure I remember seeing some systems for sale in the states which uses an electric drive to spin up the turbo and to keep it spinning when its not boosting. Not at all sure of the details. May have been in one of the dodgy 'Custom Compact Car' magazines next to an advert for an exhaust flaming device (I think my 7 has that built in)

[Graham Perry]

Robbie, I think you are correct, I remember seeing something as well.

[Joe 90]

The F1 boys came up with a bunch of ideas to counter turbo lag in the late 80's.

 

#1 Valves in the intake bypass the compressor so that it doesn't slow down when the throttle is closed.

 

#2 Inject fuel into the turbine to turn it into a mini jet combustion chamber. If allowed, inject some nitrous too.

 

The problem with the other ideas is the amount of power required, the high rpm required (over 100,000 rpm), and the high temperatures make other solutions pretty difficult.

 

SEP field working, not spotted in 102,400 miles. Some photos on webshots, updated 10 June

[robbieclark]

My lancia integrale evo had a sort of recirculating wastegate that fed the dumped air back into the turbine side(or whatever its called) to keep it spinning - if I understood it correctly