Couple of theories on the diesel cool running problem?

[Richcl]

When my automatic diesel had a ran cold running issue, it was always on cold start, i.e. using more fuel and holding lower gears in the auto box to encourage warm up. It used loads more fuel, 5mpg less.

[kaiser]

There is given a range of temperatures where the engine is most efficient! However, just getting it hotter like a turbine is not it.
As said, certainly for a petrol engine, there is an additional amount of fuel injected if the engine if deemed to be cold.

I am not sure if this is also the case with a Diesel, but it could well be.
So a faulty sender or genuinely cold engine would increase fuel consumption, but only because the computer deems it prudent. That is not an efficiency issue as such!

[clf]

Originally Posted by kaiser

The Rankine cycle is used to describe the workings of a heat engine like a steam turbine, and in short says that the best efficiency is obtained by maximizing the temperature (energy) difference between incoming and outgoing fluid. Thus as high an operating temperature as possible into the machine followed by the best cooling once at the end of the cycle. In other words the maximum extraction of energy!

The analogy here is not there! A piston engine does not automatically get more and more efficient with increasing operating temperature. There is no scientific basis for that. The engine gets most efficient when it looses the least heat and thus converts the most to mechanical energy. A cold running engine like the Diesel is actually good, as obviously the engine is not cold because of excessive cooling, but because it converts a high proportion of energy in the fuel into mechanical energy.

With a diesel, I always thought there was an optimum temperature, because diesel ignites by the heat generated by its compression. More heat would mean less need for as much compression. Less compression would mean less mechanical energy etc. So by the reverse, by having more residual heat along with the heat created by the compression, would create a bigger bang, and more mechanical energy.

[T-Cut]

Originally Posted by HarryM1BYT

Partially correct, but there is still a temperature at which both a diesel and petrol engine operates most efficiently

We're all agreeing the same thing, but arguing over semantics. Running the M47R hotter than is typically observed here will improve its fuel efficiency. This has been demonstrated and the numbers speak for themselves. A diesel engine isn't a petrol engine isn't a gas turbine, but it's the same physics applied as best we can.

TC

[Elliott]

By virtue of wanting more power from a diesel by the populous, manufacturers have left indirect diesel engines behind and moved to direct injection. Then add common rail into the mix (running up to and beyond 1600 bar pressure in the system ) we have the ability to preheat the combustion chamber by squirting a smaller amount of fuel into the combustion chamber a long time before the main injection squirt to start warming the combustion chamber for the main squirt of fuel for combustion just before TDC, we also use this pre-squirt to quieten down the combustion knock familiar with direct injection diesels.

When starting from cold the pre-squirt time (or pre injection) can be lengthened by injector opening times much like a petrol and the injection timing is altered. (unlike old carbs we alter the fuel injector opening time for a cold engine) Diesels have never enriched the mixture just altered the injection timing to advance for cold start. (Now days cold start glow plugs, very old days a burning rag in the intake manifold lol)

However on cold start much of the heat produced is used to warm up the combustion chamber and can take a lot longer than you realise, ( hold a blow torch on an old cold engine block and see how long it takes to warm up.

The question here is why doesn't it stay warm. The thermal properties are similar to a petrol hence with heat in the cylinders we require piston rings to allow for expansion of the aluminium pistons for the sealing to stay above a minimum level req'd for compression ignition. unlike gas turbines.

One of the main problems with this engine is that it was designed to sit north south in the engine bay, so the rear cylinders would sit in a nice warm bubble with manifold heat helping to keep it warm in the engine bay. But in our 75/ZT set up it is mounted east west across the engine bay, with whole of one side totally exposed to the incoming cool air, and canted back as well so even more of the engine is exposed, with any heat produced by the exhaust manifold and turbo is blown straight under the car at the rear of the engine bay.

There will never be a definitive solution as other manufacturers such as Peugeot and VW for example have designed their engine and engine bay's to trap the heat specific for FWD application.

As much as I to say it, eliminating the EGR which is used to cool the combustion chamber by means of a cooler burn with less oxygen, may in this case exacerbate the cooling of the combustion chamber too much. It would be interesting to know if any one who has bypassed the EGR has noticed the engine running any warmer, Another way would be to some how reduce the cooling system pressure in the winter, as we know a pressurized cooling system will boil at a higher temp about 120-130c . reducing the pressure will bring the boiling point down and potentially make the engine run in a hotter coolant range.

Just thought I would throw my Tuppence in sorry if I went on a bit

[T-Cut]

Originally Posted by Elliott

- - - as we know a pressurized cooling system will boil at a higher temp about 120-130c . Reducing the pressure will bring the boiling point down and potentially make the engine run in a hotter coolant range.

You really need to explain this to be considered a viable proposition.

TC

[Elliott]

Guess I was "spit balling" as they say, but was thinking if you could reduce the pressure in the system the water would boil at normal 100c, so instead 65c being half of 130c it would over 2/3 of 100c. But then the temp is what it is, its just the boiling point temp is raised as the pressure increases. So in the true spirit of "spit balling and idea bashing" this would be a no go as the problem is not one of these diesels running at 90c plus.
I humbly withdraw this idea.

[T-Cut]

Originally Posted by Elliott

I humbly withdraw this idea.

No need for humility. We all get brilliant flashes of inspiration - only to have them snuffed by the cold light of reality.

TC

[clf]

Originally Posted by T-Cut

No need for humility. We all get brilliant flashes of inspiration - only to have them snuffed by the cold light of reality.

TC

..... which is how this epic thread started lol

[T-Cut]

Ha! love it!

TC