Team-BHP - Why are V8 diesels a rare breed?

Think about it, one can buy (internationally) a V8 petrol car for as low as $30,000 (e.g. Mustang). Purists insist that an S Class or 7 series ought to be a V8 only. V8 for the unreal refinement, reserve power (at all times) and performance.

Question is : Why aren't there any V8 diesels? The biggest diesel that an S Class is sold with is a S420 CDI 4.0 liter V6, while the biggest oil-burner 7 series is a 3.0 liter I-6 (740 ld). Why are big diesels topping out at 6 cylinders?

Compare that to the petrols : Too many 6+ cylinder options with the S Class : S450 (4.6 L V8), S500 (5.5 L V8), S600 (5.5L V12), S65 (6.0 L V12). Even the BMW 7 has the 750 (4.4 L V8) and 760 (6.0 L V12).

Sure, the V6 diesels are powerful enough. But then, so are the V6 petrols. Clearly, there is a market for king-size engines in the uber luxury class.

So again, why are V8 diesels a rare breed unlike V8 petrols?

audi Q7 also has 4.2 V8 TDi.

it produces a massive 1000nm of torque. it can actually pull planet earth with its tow hook.

such huge amount of torque is not easy to control, it can twist a monoque chassis.

i remember an episode of top gear where jeremy raced a V8 4.2 liter petrol BMW 5 series against a straight 6 3 liter BMW 535D.

the pertrol version was barely able to defeat the much smaller diesel.

and also that jeremy mentioned that BMW says the diesel engine of 535D ( a 3 liter six cylinder) has so much power in top end that they had to de-tune it.

just think of a 4.2 liter V8 TDi, it will be insane.

I think the problem is torque.
With a V8 diesel, to get the benefit of tech and ample power you would have 1000Nm type of torque. A little too much for any luxury sedan.
V6 diesels currently produce more torque than the torquest V6 petrol.
Moreover, until recently, diesel was more of a mileage machine, for high free revving power it was the V8 petrol.
but winds of change are blowing, and soon V8 diesels may become a reality

Torque is a big problem. A v8 will produce more torque than the cars can handle.
sure there are a lot of diesel V8s in the market, but are used in big SUVs and pickup trucks. where there is room to put bit transmissions and the weight of the vehicle can be used in favor of putting that torque to use and not just seat there spinning the wheels.
And than there is the image issue, and the mind set that petrol is always be faster the diesel car.
The last is the sound one buys a V8 [even in big car like 7 series and S class] for the sound and not the diesel clatter, even with high tech Common rails a diesel is a diesel.
By the way only Bmw and Merc have the 6 cylinder diesels only, most of other companies including Audi,VW,ford,Chevrolet,etc have big diesels, V8,V10 also.

The Toyota Landcruiser too sports a V8 diesel engine.

Now if you look at the power delivery profile of this engine, its obvious that it will go into a mature chassis and a large one at that. Engine Weight per cc etc.

Torque & BHP stats for a V8 diesel will remain a high performance domain, including trucks and buses and tractor- trailer power units.

You can have light V8s petrol engines, yes, but diesel involves a heavier chassis to cradle the engine.

I think its weight which is the culprit.

Diesels run much higher compression ratios compared to a petrol, and hence need to be build extermely hardy. This adds a lot of weight. Hence a large bore and stroke V-6 could be that compromise in sedans.

As 2 bhpians pointed out,..larger ladder on chasis vehicles are the most likely to sport and more importantly support the weight of a V-8 diesel.

Quote:

Originally Posted by GTO (Post 1592776)

Think about it,
why are V8 diesels a rare breed unlike V8 petrols?

As engine capacity increases, more refinement is needed in the engine and since Petrol engines are far more refined than diesel engines, petrol engines are preferred,

Now a question one may ask is Diesels can be refined too, isn't?

Well yes, but the R & D should get enough budget for it..

Diesels have huge torque and lesser horsepower, to go faster one needs lots of power, unfortunately diesels cannot provide it.

Or else, there should be a xx speed transmission to control and balance the torque, which will be not practical.

Imagine xx speed manual transmission, one will spend more time in shifting than enjoying it..

We should also remember that if this could happen, why didn't Enzo Ferrari implement it?

Keep revving your ride...

good question!

The technical reasons, i'll leave to other more knoweledgable bhpians :) .

I'm quite sure if Audi can win LeMans in a diesel, the boffins can design a road car to handle all the power and torque of V8 & v10 diesels. .

Diesel supercar anyone?

This article may explain why Merc and BMW do not have V8 diesels in their lineup.

And I really agree with this explanation as the people buying diesel will mostly be concerned with fuel economy that anything else.

Quote:

Originally Posted by GTO (Post 1592776)

Think about it,

the V6 diesels are powerful enough. But then, so are the V6 petrols. Clearly, there is a market for king-size engines in the uber luxury class.

So again, why are V8 diesels a rare breed unlike V8 petrols?

Found some interesting info, here it is..

Audi's stunning V12 bi-turbo diesel engine is truly the star.

Audi launched the direct-injection "TDI"-type engine in 1989. Today half of all new Audis carry a TDI powerplant, with the top of the line production diesel engine a 4.2-liter V8 offering 326 hp and 650 Nm torque.

The all-aluminum V12 bi-turbo is a huge leap forward. Although limited by the Le Mans regulations to a maximum displacement of 5.5 liters, a maximum boost of 2.94 bar, and a restricted air intake, the engine still develops 650 hp, and a stump-pulling torque of 1,100 Nm.

The experience for drivers and spectators used to screaming small displacement V8s will be unique - the diesel power band lies between 3,000 and 5,000 rpm. Equipped with diesel-particulate exhaust filters, the engine is said to be so quiet that the drivers can't hear it when the car's up to speed, while spectators and photographers will miss not only the howl of the engine, but also the off-throttle exhaust flames that have become a feature of night-time endurance racing.

Keep revving those diesels..

Note from the Team-BHP Support : Please note that it is now imperative for you to upload pictures directly to the Team-BHP Server. Click here to view our simple help article on uploading pictures.

Quote:

Originally Posted by pb10gagan (Post 1592821)

audi Q7 also has 4.2 V8 TDi.

Possible explanation:

V8 diesels have the problem of excessive torque. Add to that the fact that transmission has to be strong enough. And with the current technology, the 6 cylinders are doing the job well. They manage to give out sufficient performance from 6 cylinders.

IMO, V8 diesels are best suited for SUV where they really need all that punch to move the massive vehicle. Its not an easy task to handle the massive torque.

But the question you have put up is very interesting. It keeps me wondering why cant we have more V8 diesels.

The Audi V8 gives our750+ Nm torque, not 1000Nm. Its the 6.0 V12 diesel that gives out 1000 Nm torque, and that too its limited as the transmission could not handle more.

Audi V8 diesel has bore x stroke 83 x 95.5 mm, 4134cc, 16.5 compression ratio, twin turbo and two intercoolers, 322 bhp and 77.55 Nm of torque. That is, 18.759 Nm/ltr! Very nice for such a large engine.

Audi V12 takes the game further.
This was a development of the motor used for R15 in LeMans.
Bore X Stroke : 83 X 91.4mm
5934cc, 16.0 compression ratio, 111 Kgm of torque.
Two turbo ( water cooled ), two intercoolers, water cooled alternator, four radiators.
493 bhp, 18.XX Kgm/ltr.

Audi has used V10 in 2004.