Do diesels need compression?

[Burnerman]

Well, I always thought they did.
Just done a 'refurbish' on an old Benford dumper - single cylinder Petter diesel with an AVA engine. (Direct injection).
I couldn't find a new piston and barrel - this thing was so down on compression that the motor would turn over on the crank handle whether the decompression lever was on or off - seriously it made no difference, the wheeze between piston and barrel could be heard 100 yards away.
I duly informed the owner that it was knackered, and we would need a new piston and barrel to go any further and it would never run again like it was. (I had fitted a new injector by then, as the old one was seriously damaged).
The owner appeared, and being built like a bear cranked the engine over....it started up immediately!
Believe me, I reckoned the compression ratio was about 6:1 - how it started I'll never know
John

 

[Peter.N.]

Just brute force I would think John. If it could be turned over fast enough to fire before all the pressure escaped it was probably going fast enough to continue running. Some of the old DI tractor engines would run with amazingly low compression once you got them started. As the internal temperature rises they don't need nearly as much compression as they do to start from cold. You also of course get an improved seal as the oil starts getting splashed about.

Peter

 

[Avocet]

Also, they have lots of stored energy in the flywheel (I assume only single cylinder)? So if you "sneak up on" them and get a decent speed up before the compresion stroke, you can sometimes carry them through it on the energy in the flywheel and then they fire. It's presumably a very low powered engine, so not THAT much work required. I have a 12 horse single cylinder boat engine that can be cranked by hand (though it does have a decompressor and it does make a bit of a difference)!

 

[Burnerman]

Thanks for your comments, gentlemen
This ancient old beast has a decompression lever, and believe me there was absolutely no difference in compression if it was used or not
I suppose I can get some comfort that the owner (muscles like Popeye) was able to crank the thing so quick it was almost bouncing off the ground
John

 

[Avocet]

Could it be that the decompressor was cream-crackered, rather than the compression was very low? As an aside, gould you get at least some of it back by shoving a honing tool down the bore and throwing a new set of rings on?

 

[Burnerman]

When the machine came my way, it wouldn't work at all - initially I thought it was purely a compression problem but then I realised that the injector wasn't making the characteristic 'chirp' sound when it opens.
So, off with its head, ground the valves in, checked what I could (piston rocking cheerfully in the barrel) and it was at this point that I found I just couldn't get a piston, barrel or ring set for any price.
I was able to get a gasket set, though!
I noticed that the head gasket had gone (flat copper ring type) and I put that down to a birds nest blocking the air cowl to the head the huge ball of straw had actually caught fire at some point.
The decompressor was lifting the exhaust valve off its seat ok, and had clearance in the run position.
The injector was reconditioned (£40) at a local specialist and that worked perfectly after.
So, thats the story - considering the amount of Petter AVA 1 engines that have been made I would have thought new barrels / pistons would have been plentiful, but no.
John

 

[Peter.N.]

Perhaps they never really wear out John

Peter

 

[Avocet]

I think so, in general. That said, my last petrol car was scrapped at 248,000 miles.

 

[Peter.N.]

All engines seem to last a lot longer than they did but diesels still have the edge. I sold a Citroen xm diesel at 292.000 miles and have another that has done nearly as many. I bought my present 406 Hdi with 188k on it and have done another 10k without a hitch and it still runs beautifully, there are a number for sale with over 300k on them - I don't think the newer ones are so good though, which I am convinced is down to the extended oil change intervals.

Peter

 

[gblades]

I think the engine itself has a longer life but with most of them now having turbos and a lot having DPF I suspect diesels may well start being scrapped earlier due to these high cost parts failing and making it uneconomical to repair.

 

[Peter.N.]

I think this situation could be reversed by changing the oil twice as often. I have never had a turbo fail on a diesel in half a million miles of motoring but have always changed my oil at 5,000 miles, the new ones proabably need it done less but a maximum of 10,000 miles.

Peter