Why do turbo-diesel engines have such high torque?

[bjdm151]

Yea its me Jim,

I figure if i suck up now it might get me a fifty cent raise when I come back. HA

[SaabJohan]

The diesel is NOT used because it has such high torque, transport heavy loads, acceleration ans so on is dependant only on power, how it's developed is a minor concern. To have a broad torque curve is however a different issue, but even truck diesels have curves that are that broad. That's why we find as many as 14 gears in some heavy trucks (or perhaps even more).
In the recently developed racing engines the high torque is rather a problem since there is difficult finding a suitable transmission for them, which also becomes heavier with increased torque. Of that reason engineers are now chasing higher engine speeds.

The main reasons why diesels are used:
1. Diesels offer a high engine efficiency. Why? This has several reasons, the high compression ratio, the lean running and at part throttle they don't have any throttle plate causing high pumping losses. The high engine efficiency which typically peaks at above 40%, but over 60% in large diesels, gives a low fuel consumption and low CO2 emissions as they are related to the fuel consumption.

2. Diesels run on a safer fuel. Diesel isn't as flammable as gasoline, throw a burning match in a drum of gasoline and you will have a big fire, do the same with diesel and the match is most likely put out. Why you ask! Well, diesel typically boils between 180 and 370 degC, while typical gasoline boils between 25 to 215 degC, so with diesel there aren't that much flammable vapors at normal temperatures that can ignite and burn (this is called flash point). This was probably the main reason why diesels found use in for example submarines, where gasoline engines were first used.

3. Diesels last long. Diesels have shown excellent performance in durability not matched by gasoline engines.

But diesels have downsides also, for example:
1. Diesels use high combustion pressures, low speed and require more air compared to they gasoline counterparts. This adds up to poor power to weight, and power to size ratios.

2. The initial cost of a diesel engine is larger than for a gasoline counterpart.

3. Emissions of particles and NOx are high, some diesel fuel does also contain quite a lot of sulphur (mainly a issue for larger diesels) which isn't just bad to the engine it also causes emissions of sulphuric acids (or more correctly sulphur oxides which when mixed with water become sulphuric acid).

You can probably see where I'm going with this, diesels are used where a higher weight/size can be accepted and running costs are the largest issue.

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Why are diesels producing a such high torque then?
As I said earlier diesels don't have BMEP's that are that different from turbocharged gasoline engines. BMEP is Brake Mean Effective Pressure.

BMEP = (2Pi*torque*2) / volume

or rewritten

torque = (BMEP*volume) / (2*2Pi)

If it was a 2 stroke engine we should muliply with 1 instead of 2.

As you can see the torque of an engine is given by the mean effective combustion pressure in the cylinder and the cylindervolume.

Please note that a longer stroke does NOT increase torque, this seems to be a common misunderstandning that a longer crank throw should somehow increase the torque, this isn't the case since, with a given volume, any gain in stroke will result in a loss of piston area and these two equal out eachother since a loss in piston area will result in lower force on the con rod and the loss in stroke of crank leverage. If I remember correctly this issue have been up before and explained.

So if torque is only dependant on BMEP and volume, why does the diesel deliver such an impressive torque. Well, to answer that question i think I need to explain more how the diesel engine works.

The diesel engine operate with an excess of air, torque is controlled by the fuel to each combustion. The ideal diesel process is a constant pressure or mixed constant pressure and constant volume process (the ideal otto cycle is an constant volume process). Modern direct injected diesels (hereafter refered to only as diesels) inject the fuel direcly in the cylinder with a centrally placed high pressure injector, during intake and compression there is only air in the cylinder. During the compression phase the temperature rise in the cylinder to above 400 degC which is the self ignition temperature for diesel fuel. At around 15 degrees before top dead center the fuel injector starts inject fuel, this is done with pressures around 2000 bar which gives a very fine mist. Because of the temperature in the cylinder the fuel will start to vaporise and when the vapor comes in contact with oxygen in the cylinder it will, after a short reaction time ignite and burn. Diesel is then injected for a duration of about 40 degrees, which means that the injection typically ends 25 degrees after top dead center. This injection period can be extended further but then a larger portion of the fuel will remain unburned or burn in the exhaust manifold resulting in a high fuel consumption, high exhaust temperature and black smoke.
As you probably understand the diesel does't have a regular flame front as the gasoline engine has, where a flame travel from the source of ignition and outwards. Of this reason the diesel can't knock as a gasoline engine can but its combustions also takes a little longer times which decrease engine speed.

These caractaristics make the diesel suitable for low engine speeds, and high boosts. The high efficiency causes not only the fuel consumption to decrease, it also increases the combustion pressure, but diesels also run with air excess which decrease the combustion pressure. This gives the diesel BMEP's similar to a turbocharged gasoline engine. The thing is that we rarely see gasoline engine with similar values in boost and displacement, and if we find any they will use higher engine speed causing that the power is much greater in comparison with the torque since power=torque*angular_velocity (angular is rpm, but in rad per second instead of revolutions per minute).

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There have also been posted some more and some less correct info about diesel fuel.

As mentioned diesel isn't measured in octane but in cetane, and cetane describes the fuels "readiness to ignite". Typical diesel fuel is rated at 50 or somewhat higher.

Diesel have a slightly lower energy content, at least if we relate it to mass. Diesel have an energy content of 42.5 MJ/kg, compared to premium gasoline which has 43.5 MJ/kg and regular with 42.7 MJ/kg. But gasoline has only a specific gravity of .715 to .780, while diesel has .815 to .855 so diesel contains more energy per liter of fuel.

The stoichiometric air fuel ratio for diesel is 14.5 kg air per kg fuel, slightly lower than gasoline with around 14.7.

[Evil Result]

What are the main problems that are faced if you wanted a diesel engine operating at high rpm with performance relitive to a gasoline engine?

What could be done to allow a diesel engine to operate at those speeds and performance characteristics?

How much of a difference in flame front velocity are we dealing with between gasoline and diesel... and how much does this effect the force delivered upon the piston at any rpm? or is the main problem the time it takes for the fuel to ignight.

what if there where more injectors instead of one percylinder to increase fuel delivery yet maintain a proper fuel mist.

Here is a base id like to know if diesel would work on....

3.600" bore
3.000" stroke
6 liters
9500 RPM redline

[CBFryman]

^^forgot to add that efiency part into it. but ya Rudolf Diesel origianlly designed the diesel engine to be a highly efficient. the fule originally used was Heavy oil (basicly a very light motor oil like fule) or vegitable oil. yo ucan still run vegtable oil in your diesels of today and get better efiency and better power than with regular diesel (incase you havent noticed diesel is very oily compared to gasoline, and as a matter of fact GM will allow you to run up to 75% diesle and 25% vegtable oil in your engine and not void the warranty. but yo ucant just use any vegtable oil. it too has to be refined). the reason the diesle engine later used "diesel" was because both gasoline and diesel used to be thrown away at the refinaries so it was cheper. refinaries used to only look to make kerosien and lubrication oil. now gasoline is one of the most used products coming out of the refianry.

[bjdm151]

Evil,

I believe one of the factors that limits engine speed is the slow burn of the fuel used. I'm not sure of the geometry of of most diesel engines, but i'm guessing (somebody correct me if i'm wrong) that they have a low raod stroke ratio, wich increases the accelaration and speed of the piston at a given rpm. Too fast of an aceeleration and stuff goes boom.

Gasoline is made up of hydrocarbons with shorter chains, which makes them easier to burn than gasoline, so more heat is needed in a diesel engine (hence the higher compression and things like glow plugs and intake charge heaters.) Also, in a gasoline spark engine, fuel enters the piston on the intake stroke. On a diesel, fuel enters in the later part of the compression stroke when the piston is moving up. Deisel injectors also operate at much higher pressures thn gasoline injectors (enough to cut a hole in your hand)

I don't really feel like doing the math right now , but i don't think you could get a diesel engine up that high in rpm (9500)

Somebody fill any blanks i have left

[Evil Result]

well i'm not talking about converting a Gasoline engine over to diesel....purely deisel design.

A shorter Rod to stroke ratio effects the piston acceleration and deceleration at the top and bottom of the stroke which means it also slows the piston speed in the middle of the stroke.... if most diesel engines use a shorter ratio, this makes sence because it would cause the air to compress faster and increase heat.

But, it dosen't matter as much in a diesel as in a gasoline engine... overall it would be better to have a longer ratio in any form of engine because it reduces cylinder wall loading which reduces wear and reduces peak piston velocities improving engine life.

Just keep increasing the compression ratio if the adjusment to the ratio effect the air combustion temperature, or get more boost

does a smaller bore improve the burn characteristics of deisel?

The largest diesel engine in the world which i'm shure we have all seen operate at 55% efficiency which is the highest known for commercial production I belive.

[SaabJohan]

The Ricardo V10 turbodiesel gives its maximum power of 600 hp at around 6000 rpm, I don't know of any diesel using higher revs than that. But I do know of truck engines using more than 4000 rpm, pretty impressive since we talk about diesels over 10 litres with a stroke in the region of 140 mm.

Since the combustions in a diesel is slower I would go for a higher rod/stroke ratio, but that will make the engine very high.

A diesel doesn't have a flame front like a gasoline engine, the fuel burn as I have described in earlier posts. There are problems with getting an efficient burn at higher engine speeds. To get an effcient combustion at higher speeds the fuel must vaporise and burn faster, releasing more energy in a shorter time. Ricardo have for example developed an high speed combustion engine for their V10 engine. But a really powerful diesel must also have head and block in cast iron if you want it to last, piston crowns needs to be steel.

[Evil Result]

Hum, that may be the issue... every high output Diesel is always bulit with a large displacement and large stroke...with a large stroke you can't rev an engine at high RPM because the bottom end would blow out and the displacement is so large that injectors couldn't supply fuel fast enough(mabey not?) to each cylinder at high RPM.

so what if we used a 3 inch stroke with a 3.6 inch bore V12.... you would have half the displacement per-cylinder that the 10 liter V10.

also you could probley get away with a non-heavy duty constriction of the cylinder wall, head, and pistion because your not producing as much power in each cylinder because of the lower displacement.

I noticed that diesel engines commonly use a very interesting quench area bulit into the piston head.

[SaabJohan]

The Ricardo diesel is just 4.6 litres
http://www.mulsannescorner.com/ricardojuddv10.html
I would guess the bore stroke is somewhere around 80 x 90 mm.

The truck engine using over 4000 rpm is a modified for high performance (it's not used in trucks), it has a displacement of 14 liters and crank out 3000 hp.

A diesel must be heavy duty, if we looks at truck diesels they are always made from cast iron, piston crowns are sometimes steel. Aluminum wouldn't last as the combustion pressure can be above 200 bar.

Diesels are using a longer stroke than bore as this suits their low speeds and that it's needed to reach the high compression ratios.
Diesels have the combustion chamber placed in the piston, the crown of the pistons goes up so it meets the cylinder head. The design are used to create a swirl flow for a more efficient combustion.

[curtis73]

A couple main factors in their RPM limitations...

1) hydraulically controlled mechanical injector pumps
2) high compression
3) the type of cam profile used to make efficient operation of the setup (no throttle plate and turbo are the main design factors)

The first is being counteracted by modern electronics, the second can only be lowered so much before eficiency drops off fast, and the third can be crutched with VATN turbos making boost way up high as well as down low to supplement the "small" cam, but then you run into extremely inefficient turbo outputs with high heat. Yanmar diesel is currently making some interesting engines with these things in mind.

On an opinion note; I really must say that although the things being said about the negatives of diesel are all valid... if we were in 1975. The automotive world has us all fleeced into thinking HP is what we drive. HP is a calculated number derived from torque. It is torque applied over time.

Anyone who's driven a newer diesel pickup wouldn't say "diesels are slow". The 215 hp in my 1995 Powerstroke felt like much more since it made nearly 500 lb-ft

All engines MAKE torque and torque MAKES hp. The only difference is where the torque peaks. Diesels peak their torque down low which yields lower hp numbers.

The VW TDI makes incredible power for its tiny size, so the 12 second 0-60 time is a bummer... but not because its a diesel. Its because its a 1.8L.

Also, anyone who doubts diesel, I have a friend with a Powerstroke diesel making 600 hp and 1000 lb-ft of torque. Its heavily modified with propane and twin turbos making 40 psi. The real benefit (in my eyes) is that its a daily driver. Modding diesels doesn't ruin its drivablity like with gas. Why? No throttles, so there is no vacuum for a load base, and they strictly rely on injecting fuel into the air that's available. Modding it just means you're increasing the potential for both but since you haven't changed its VE at idle, its the same pussy cat at idle