Diesel Engines

[DanJR1]

Can someone just explain why all diesel cars seem to have turbos? and i know that a 2 litre petrol engine would produce around 250hp in race spec so how much would a 2 litre diesel engine produce?

[ysofast]

A turbo gas engine is a neat gimick, worked well in the 80's and 90's but normally aspirated gas engines produce nearly as much power and drive better. Diesel motors are popular (usually as turbos) because fuel prices are better and you can boost a common rail motor 4-5 times as much as you can a gas motor. The torque to horsepower ratio is very high in this type of motor so it gives the feel of a much bigger engine without the taxes, weight and or extra fuel consumption. Recent innovations make the td's even more attractive ie less soot and noise, vibration.
Who knows what the power limit would be in race trim. Not far off gas motors using premium fuel(can't get close to higher octane fuels tho)

[Pug620]

turbo deisels are great fun as i have a 150 bhp pocket rocket and can not get less than 40mpg have been told i can 180 but its onlys FWD

Has any one ever built 4x4 deisel in a performance car form, cause i think this would have to be then next stage as they have so much torque you would need the 4x4 to handle any more power?

Marty

[1200Datto27]

Diesels run turbos as to greatly improves the combustion of the fuel (remember, they use a Compression ignition, not a spark) and the efficiency of the engine.

Power most likely wouldn't be as much as the equivalent Petrol engine (depending on allowed boost pressures) but the amount of torque produced would be a lot greater.

[DanJR1]

surely it would be possible to gear the gearbox to use the torque so that the performance is the same as a petrol?

[Dave Brand]

Originally Posted by DanJR1

surely it would be possible to gear the gearbox to use the torque so that the performance is the same as a petrol?

That's exactly how it's done! A gearbox is, in effect, a torque multiplying device.

The measure of performance is BHP, NOT torque! Torque at the crankshaft is irrelevant; it's what's happening at the driven wheels that matters. To accelerate a car, a force is required at the rear wheels (Newton's Second Law of Motion) - in addition to that, a force is required to overcome rolling resistance, drag, etc. Take two cars, moving at the same speed; one has a diesel engine, turning at 2,000 rev/min & producing 200Nm torque, the other a petrol engine turning at 4,000 rev/min & producing 100Nm torque. The force at the rear wheels will be the same for both, because of the effect of the gears between crankshaft & wheels. It doesn't take a mathematical genius to work out that both engines are producing the same bhp!

Compare performance figures for similar cars with petrol & diesel engines of similar power & you'll see that 0-60 mph & top speed are more or less the same, despite the diesel producing much more torque. In-gear acceleration times may be different, often in favour of the diesel, but that is a function of the shape of the torque curve, not absolute torque values.

[dtype38]

Not convinced its as simple as that Dave. A 100hp motorbike can accelerate very quickly but can't pull a car trailer. A 100hp diesel tractor can pull half a dozen car trailers, but accelerates like a snail. Thie difference is all in the torque capability of the engine. The gearing is important, but I think the length of the crank throw and duration of the combustion stroke have a lot to do with how quickly a certain mass can be accelerated.

[Dave Brand]

Originally Posted by dtype38

A 100hp motorbike can accelerate very quickly but can't pull a car trailer. A 100hp diesel tractor can pull half a dozen car trailers, but accelerates like a snail.

Gear a 100bhp bike low enough & it will pull like a tractor!

Thie difference is all in the torque capability of the engine.

So why do diesel cars need gearboxes?

The gearing is important, but I think the length of the crank throw and duration of the combustion stroke have a lot to do with how quickly a certain mass can be accelerated.

It's an old fallacy that long-stroke engines produce more torque than short-stroke engines. For a given cylinder size & the same BMEP (Brake Mean Effective Pressure) the torque & bhp will be the same to within a few percent over a wide range of bore/stroke ratios. The rate at which a mass can be accelerated is determined solely by the force available to accelerate it - unless Newton got it wrong!

I've designed a few machines in the course of my working life. In specifying a motor the criterion I've used is power, never torque.

[dtype38]

Dave, if you've specified motors then you know that all sorts of stuff works in theory but not in practice. eg in theory there's no reason why a motor of a given power could be very long and thin, or very short and pancake shaped. Such things do exist, but for the most part to maximise efficiency and minimise manufacturing cost, then motors tend to have a characteristic length to diameter ratio. In the same way, engines are either produced as undersquare bore/stroke where torque output is more important than power, and oversquare if the reverse is the case. A Kawasaki 100hp bike is very oversquare because its not expected to haul large loads so doesn't need to produce much torque. You are correct that in theory it could be geared to, but the diven gear would be far too large to fit into a realistic gear box and multiple reduction gears would be uneconomical. On the other hand, a 100hp Harley Davidson is very undersquare and may not be able to accelerate too well, but it can get a caravan moving if needed!

Oh, and diesel cars need gearboxes, because no diesel engine has a working rev range big enough do 1-100mph (say) without one. On the other hand you probably wouldn't find a gearbox on a small earth mover because it only has to do a 1-15mph range.

[dtype38]

Oh, and DanJR1... getting back to the original question. Diesel engines need a lot more compression than petrol engines because they just use the compression itself to ignite the fuel. That tends to make them very noisy and not very compact because they need a long stroke crank. If a turbocharger is used it effectively "pre-compresses" the inlet air so that a lower compression engine can be used. This makes it both smoother and more efficient and therefore worth the extra complication when competing against more refined petrol engines in modern cars. As the power bit of the question, I'd have to get out a Mercedes brochure to tell you that

[Dave Brand]

Originally Posted by dtype38

Oh, and DanJR1... getting back to the original question. Diesel engines need a lot more compression than petrol engines because they just use the compression itself to ignite the fuel. That tends to make them very noisy and not very compact because they need a long stroke crank.

Compression ratio has nothing to do with stroke! It's simply the difference between combustion chamber volume at BDC & TDC. Most car diesels nowadays have the same bore & stroke as a similar-sized petrol engine. The noise a diesel makes is an inherent part of the combustion process.

If a turbocharger is used it effectively "pre-compresses" the inlet air so that a lower compression engine can be used. This makes it both smoother and more efficient and therefore worth the extra complication when competing against more refined petrol engines in modern cars.

The main purpose of a turbo is to get more air into the engine; the more air it breathes, the more fuel it can con burn, the more power it can produce.

[dtype38]

Originally Posted by Dave Brand

Compression ratio has nothing to do with stroke! It's simply the difference between combustion chamber volume at BDC & TDC. Most car diesels nowadays have the same bore & stroke as a similar-sized petrol engine. The noise a diesel makes is an inherent part of the combustion process.

I'd be interested on some actual figures on that. Say a 1.6 VW golf petrol and diesel. If what you say is correct then the diesel would have a much smaller combustion chamber. I thought the advent of similar bore and stroke was directly connected to the more general use of turbos on the diesels.

Originally Posted by Dave Brand

The main purpose of a turbo is to get more air into the engine; the more air it breathes, the more fuel it can con burn, the more power it can produce.

Or... the main purpose of a turbo is to usefully use a lot of the wasted energy going out the exhaust valves to produce a much more fuel efficient engine. Depends on what you're using it for

[Dave Brand]

Originally Posted by dtype38

In the same way, engines are either produced as undersquare bore/stroke where torque output is more important than power, and oversquare if the reverse is the case.

That, I would say, is a half-truth. The trend to more oversquare engines came about in the search for more power. A bigger bore allows much bigger valves, thus allowing the engine to flow more air, while the shorter stroke keeps piston speeds down at higher engine speeds.

A Kawasaki 100hp bike is very oversquare because its not expected to haul large loads so doesn't need to produce much torque. You are correct that in theory it could be geared to, but the diven gear would be far too large to fit into a realistic gear box and multiple reduction gears would be uneconomical. On the other hand, a 100hp Harley Davidson is very undersquare and may not be able to accelerate too well, but it can get a caravan moving if needed!

The V-Rod, probably the only Harley with anything like 100 BHP in stock tune, has 100mm bore, 72mm stroke. Very undersquare?

I don't know about 100 BHP Kawasakis, but my Honda CBR600 has a very flexible engine, which pulls well from 3,000 rev/min right up to 14,000! (it will pull happily, if not very enthusiastically, from around 1,500!) The torque's there, it just looks low on paper because high-revving engines produce relatively more BHP then torque. All engines need to produce torque.....there's no BHP without it!

[dtype38]

Ok, from a motorbike point of view over and under square are a bit relative. What's the bore stroke on your bike?

[Tim Falce]

Originally Posted by dtype38

Ok, from a motorbike point of view over and under square are a bit relative. What's the bore stroke on your bike?

65 x45.2mm

[dtype38]

Originally Posted by falcemob

65 x45.2mm

I'll take it you looked that up as you don't actually own one. So bore/stroke would be around 1.44. The Harley Dave referred to wasn't the one I was thinking of (I thought they were mostly approximately square) but the fact that it's a twin makes a difference as well. Unless CBRs are twins these days, in which case I might just go and hide under my bed now. Would account for Dave thinking it has good pulling power though.

My race car has 87mm bore and 106mm stroke... I think you would agree that's undersquare. My max lbft is a bigger number than my max bhp! How do yours compare??

[Tim Falce]

Originally Posted by dtype38

I'll take it you looked that up as you don't actually own one.

Don't own one and wouldn't want to but my neighbour is a CBR fanatic so I asked him

[Eric Falce]

As a direct comparison but on a commercial theme,in the 60s Leyland produced an engine called the 680(cubic inch) producing 210 bhp,forgot the torque figuares,non turbo.Volvo used the same block but used the old crank/bore to give 600 cu inch,changed the heads,whacked on a turbo,produced 260 bhp,a lot more torque,much more economy,laid the foundation of the present day Volvo commercial vehicle success story.
Was at Brands in October driving my Escort,came out of clearways and was passed by a VW Diesel doing, half my revs,just used the torque to pull away from me,great fun to watchhrug:

[DanJR1]

does using hydrogen mean loss of power or extra weight on a car or bigger fuel tank?

[listernoble]

Petrol engines,esp race engines will not have much torque but loads of bhp at very high revs.
Diesel engines are designed with a different torque curve.
Maximum torque is much lower down the rev range,this gives more lugging power.
My Mazda 4x4 pick-up truck pulls best from around 1500rpm,but it's gutless for overtaking above 3000rpm.
The reason for this spec is that when pulling loads , as the revs drop say going up uphill ,the lugging power actually increases and this why you never see a petrol tractor or lorry!
This torque curve gives fantastic overtaking in top gear,so diesel engines could be used for racing cars,if they could be made light enough.
In fact I'm sure there is/was a diesel car race series in the UK,maybe VW?

Lister

[Tim Falce]

Originally Posted by listernoble

In fact I'm sure there is/was a diesel car race series in the UK,maybe VW?

Lister

see ecktic's post above

[Eric Falce]

Just to raise a point on torque versus BHP,my last dealings with commercials about 2* odd years ago,there was a power war going on,the British Ministry of Transport was insisting on X number of BHP per ton,this outlawed the Gardner engine for the max carrying capacity as it only produced 180 BHP,fantastic torque and even more fantastic economy.reves where rising from 1900/200 for the gardner to 25/2600 for other engines.the gardner returned 10 plus to the gallon,the best of the others,6.5 MPG. Now they are all back down to around the 2000 RPM mark,increased pulling,increased economy,so RPM and BHP are not the prime issues to get the perfomance you require,in fact I am thinking of putting one in Tims D Type!

[*J*]

Guys, i think you are losing the point here. The difference in torque and hp curves is not simply down to how square the engine is, cam profile, fuel map, port design, valve design/size etc all play major roles. Toque is directly related to horse power through rpm anyway....so your kind of arguing against yourselves here, but yes it is true that diesels tend to produce a lot of low down torque, but dont rev as high, they dont need to! This however doesnt really lend itself too well to a racing car engine, although things are changing all the time.

The noise is due purely to the vibrations associated with the combustion process for a diesel engine. nothing more. Turbos are generaly used on diesels as they can be run at much higher CRs and still be turboed and not suffer pre-ignition problems that high CR petrol engines suffer from. The higher CRs mean much higher efficiency, hence partly why diesel engines are so much more efficient.

The stroke doesnt have to limit or even govern CR, the combustion chamber volume can be altered to give desired CRs.

[Dave Brand]

Thanks, *J* - that's exactly what I've been trying to say!

[listernoble]

I think most of are agreeing on the same thing,just the wording is confusing!
We run a lot of diesel tractors and an old TVO 1950's grey Fergie which has a Standard Vanguard engine,same as they put in the early Triumph TR3's.
No,that doesn't mean you can go out ploughing with a sports car.
Lister