Why are there so many diesels of 1900cc capacity?

[scotty7]

Looking around for a new car, I noticed that a lot of diesel cars (in europe anyway) are 1900cc. Is this because of a tax break somewhere, or is it based on a 2 litre petrol block beefed up for diesel operation, or something completely different?

 

[WGJ]

I reckon, but don't exactly know, that it's a tax issue.
Most of the 'modern' diesels (common rail, high pressure injection) have their own blocks these days as the 'beefed up' petrol blocks were usually noisy as diesels and on the heavy side.



Bill

 

[PJGD]

I think that you will find that when you are juggling all the variables and compromises that go into a light-duty automotive engine design, you will find that around half a litre/cylinder is a sweet spot. Once you have developed, optimized and calibrated all the variables, it becomes convenient to build engines that are multiples of that displacement.

You can have displacements larger than ~500 cc/pot, but then you can run into NVH issues at the 4,000 rev/min plus rated speeds of such engines. Likewise, go below 300 cc/pot and it becomes difficult to fit in a conventional injector in the middle of the 4 valves, since injectors do not scale down in size, so you end up compromising breathing and/or cooling around the valves.

PJGD

 

[GregLocock]

At Austin-Rover I worked on two diesel from petrol conversions.

Neither had a hope of working except as a desperate expedient.

The cooling requirements for the head, in particular are very different.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mcmtb]

As has been said, 500cc is a sweet spot for diesel engines.

But the 1900 did originally come from the fact alot of disels were based around the equivalent 2.0 petrol. To minimise costs, similar tooling was used, which meant that things like boring jigs, which were set-up to machine bore spacing suitable for a 2.0l petrol were still used, but to have sufficient wall thickness for the higher diesel combustion pressures, they had to reduce bore diameter slightly.

That's one of the reasons why a lot of ford car engines used the same bore spacing, and instead kept the same bore spacing across many engine sizes.

 

[globi5]

In order to double the compression ratio, don't you need to reduce bore anyway (higher stroke/bore ratio)?
(And with its reduced max rpm there's probably no reason to have a small stroke/bore ratio anyway.)

Back to the 1900 cc question. At least the new Diesels sold in Europe are all over the place as far as displacement goes (1.4, 1.7, 1.9, 2.0, 2.2, 2.4, 2.7 etc.).

 

[mcmtb]

No. For higher compression ratios, you just need different pistons and cylinder head design.

On diesels combustion chamber pressures are far higher than an equivalent petrol, so the rest of the engine has to be 'beefed up' to cope with the higher forces. If you used an engine with the same cylinder wall thickness as a petrol, the cylinders would quickly fail. Easiest way to make them thicker, without having to majorly invest in new tooling was to reduce the bore size.

It's only now that diesel engines have become so popular, that the big manufacturer's can justify the expense of fully designing them from scratch, which is why they come in a wide range of sizes.

 

[patprimmer]

There is only so far you can go with cylinder head and piston design before you get to a point where you have a piston to valve clearance problem, or you have to short a cam duration to obtain the rpm to give the required power density.

There is a workable range of bore to stroke ratios for petrol engines and a different but overlapping workable range for diesel, taking into account required power density and therefore rpm, and consequences on valve size and number, airflow, piston speed, cam timing, compression ratio, forced or normally aspirated induction, injection nozzle or spark plug size and position.

I am sure I missed something.

Bottom line, like any design, there are complex multiple compromises and simple rules of thumb to try to quickly accommodate all the variables

Regards

eng-tips, by professional engineers for professional engineers
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[dcasto]

The EU taxes cars on displacement, the 1900 is a cut off point to next higher tax.

 

[globi5]

Here's an example where they maxed out the compression ratio on a 1.0 l gasoline VW engine and ended up with a CR of only 13.5.

http://www.imrt.ethz.ch/research/engine/CNG/cev

It's not quite on topic but an interesting read anyway.