Yup, the Saab design is really well explained here....

http://www.saab.com/main/GLOBAL/en/vepsilon/

....and the Mayflower design is also quite interesting....

http://www.mayflower-e3.com/tech/mn_...ail_flash.html

....but I'm sorry, the whole internal combustion thing is just getting a bit tired. Time to look towards new technologies. (The technically minded might be interested in the following article...)

http://www.benerridge.freeserve.co.uk/lis3.htm


[Edited to fix broken link]

[Edited by cletterridge - 4/2/2003 12:17:04 AM]

 

Just wondering what the technical reason was?

 

Its not that they can't necessarily, its just that because of their characteristics the max power occurs at lower revs, so there is no point reving it any higher as the power actually drops off

 

A better question would have been- why can't diesels produce their power in a similar way to petrol cars ie at higher revs over a wider band.

Also why do they tend to have higher torque- is it because most of them are turbo'd?

 

I think its mainly due to the fact that they have to run at higher compression ratios to get the fuel to ignite.

[Edited by ajm - 3/30/2003 12:28:27 PM]

 

Yep. Petrols have a compression ratio of around 10:1; Diesels are about double that at around 20:1 and therefore have to be built a lot tougher. Also, as the burn process is different (petrol has a spark to ignite the mixture, diesel reaches flashpoint under compression) there are limits to the time you can achieve complete burn and hence maximum revs.

Having said that, I wonder what effect variable cam timing would have on diesels ability to rev? (normally very mild cams)

 

The length of the stroke greatly effects their abillity to produce power higher up as well........this is also the case with stroked petrol engines, they become very torquey low down and pull very cleanly midrange but dont quite produce peak power as high up the rev range as a short stroke engine would. However they can be made to rev by a revised rod ratio. A compromise between revability and high torque would be nice

 

It's simpler than all of the above ~

Fuel (petrol/diesel/whatever) burns at a specific rate. It's a controlled burn rather than an explosion.

As RPM increases this becomes a problem - the piston is starting to move away from the top of the combustion chamber whilst the fuel is still burning.

Petrols engines obviously use a spark to ignite the mix, which you need to advance as rpm increases so the fuel starts burning before the piston reaches top-dead-centre.

Diesels use compression ignition, so can't advance their spark. In reality, latest systems allow much greater control of this, and there are often several burn stages, but you simply can't advance enough to achieve what we would traditionally consider "high" rpm.

 

Interesting mik.. perhaps they could investigate using sparks to speed the burn once it's ignited??

 

I've no idea if this is right, but somebody told me the breathing was part of it too and that the new VAG 2 litre 16V diesel will rev to 5,500 rpm. Now that isn't exactly high, but it's comparable with some petrol engines.

Fen

 

Lotus (I think) have a test bed diesel that runs an ultra high pressure injection system, I think they mentioned 250000psi. Its a 1.4litre and produces 300bhp at high rpm. High pressure makes it possible to control the timing and rev higher

Trouble is the pressure system is bigger than the engine and costs thousands, they just did it as a technology study. I saw it in a magazine but can't remember which one.

Lee

 

My 330d revs to 5000rpm which isn't that low comapare to some petrol engine. The new 204 bhp 330d should benefit from the new 6 speed box .

 

Have a look at the excellent How Stuff Works website. They have a link to why petrol and diesel engines have different bhp ratings. Good article that explains why diesel engines don't rev so high.

Chris

 

I am aware of a Nova turbo diesel that has been made to rev to over 6,000rpm with the boost wound up. No problems so far - and I hear its pretty quick too.

 

>> perhaps they could investigate using sparks to speed the burn once it's ignited?? <<

Diesels use high compression to give compression ignition so not so much a burn, but more an explosion. That's why so many diesels have that "cluck, cluck cluck", engine noise. Yes, even the 'best' ones which use devices to disguise that 'clucking'.

No point in having a high revving diesel - have to be so heavily over-engineered it would defeat the object of diesels .... i.e. low revving torquey motors combined with economy.

Finally, long before Herr Diesel developed and commercially exploited his "diesel" engine, others had previously made them when they were known as "Compression ignition" engines ...so there ...

 

A diesel engine would blow any spark plug in it to pieces as it effectively runs on detonation. Perhaps injecting less fuel to start with would cause it to detonate nice and early (effectively increase advance) then you could continue fuelling.
Edit: ~I was thinking that the injectors are actually in the cylinder like in all books on how engines work, but they're not are they, they're in the inlet manifold, so you can't add extra fuel 'mid burn'.

 

Toerag.

The latest diesels are direct injection so do inject directly into the cylinder, early indirect injection diesels had a little burn chamber prior to the cylinder. They were not as noisy but also not as economical or powerful.

Diesels such as Fords TDCi's also have a multistage injection to try and combat diesel clatter, the TDCi's actually listen out for diesel knock and adjust the burn accordingly. This multistage burn has become possible due to the very high pressures used in common rail and VAG's PD systems combined with engine management control of the injectors rather than a mechanical pump.

So yours is a good idea, but its already being done.

Lee

 

Might have been Performance Bikes. They had a very interesting article about a year or so ago about where engines are going. Must admit it seemed to be just petrol engines though. The stuff they can do on that test rig....

The next big step would appear to be pneumatic valves. The current system of using cams is a MASSIVE compromise. Having a computer control each valve on an indivudual basis not only allows you to produce a **** load more power over the rev range, but it would also keep the tree huggers happy with emissions.

I think on some test car they have/had, they had to actually *build* in a lower torque setting (i.e. less efficient engine) for lower rev ranges, because the people they got to drive it were having real problems with the power at low revs.

 

Saab have that variable compression petrol motor which gets 200+BHP from 1.6 litres, IIRC. Supposedly not too far away from production.

 

One contributing factor in diesel engines inability to rev is, I think, that fact that due to the requirement for stronger pistons than in petrol engines (high compression etc), the pistons are heavier and therefore trying to rev these pistons at high revs would put too much strain on the pistons/rods etc
Then again I could be wrong .....