Hi all,
Something just puzzle me and I cannot find a good explanation for.
Why do diesel engines get quieter and more economical when warm? What is the technical explanation behind the change in engine characteristics?
Many thanks,
G
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1) The bits 'n' bobs inside (e.g. pistons) expand to fit the 'ole proply.
2) A hot engine is more economical 'cos that's ow it works.
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I once read that diesel cars take about an hor to generate enough heat for the heating to work and hence, in winter, you have to drive about 50 miles before you have enough heat resulting in people driving around in cold cars?
The same article explained that acros Europe most diesels are sold with a special heating pump that costs about £100 to overcome this delay but, for cost reasons, most manufacturers do not include such a device in UK models?
Does anyone know whether this is true or complete nonsense? Have to say that the forum I read it in there was much discussion about comparission between European sold Avensis and UK models which, apparently, is affected by this?
I do not own a diesel, never have so have no idea but, if true, surely it is nuts having to sit in a freezing car for 50 or so miles?
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It's nuts.
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Thank you - frees up the brain by having slightly less cr*p in it.
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A friend of mine who has a diesel car does, however, tell me it takes a minute or two longer to warm up compared to a similar petrol vehicle - so there might be some difference (not 1 hour!).
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The same article explained that acros Europe most diesels are sold with a special heating pump that costs about £100 to overcome this delay but for cost reasons most manufacturers do not include such a device in UK models?
It's usually an option on European models not sure what the take up is on it though. The system can be retrofitted though it costs considerably more than £100. It's usually a 1000? option on new cars.
One company I know of which supplies these systems is Webasto (no affiliation with them). I'm sure your favourite search engine can throw up more choices if you enter "Parking Heater".
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I once read that diesel cars take about an hor to generate enough heat for the heating to work and hence in winter you have to drive about 50 miles before you have enough heat resulting in people driving around in cold cars??
LOL! Yet another diesel myth! On my D5 the temp gauge starts to move almost immediately, there's warm air through the vents within 0.5 a mile and the temp reaches fully warm within 2-3 miles quite easily, even when driving slowly through town traffic.
However, my old Vectra 1.9 cdti wasn't quite as good, but not far off.
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Cold engine requires additional fuel to maintain combustion, as it loses heat to the cold metal surrounding the combustion chamber, the cylinders and the exhaust etc. When the metal is warm the fuel requirement drops. Its yer diabatic (cold engine) followed by yer adiabatic process (warm engine, little loss of heat) in action. Awaits thumping fist from scientist.
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I was going to post something similar to FT. When the components warm up and everything expands and the oil is warmer, there's less friction and therefore noise?
I suppose its a bit like humans and animals. We need to warm up before we can work better. Athletes and reptiles are a good example.
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They are designed to run hot, hence noisy when cold.
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Yeah but no but yeah but no but.......diesel engines are still much more economical when cold than petrol ones are when they are cold. So why's that then ?
On short urban runs my diesel car is still very frugal, my petrol one drinks like a fish on those sorts of journeys. On long runs the gap is much narrower.
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In summer yaris diesel is warm - at least heater is and cold warning sign on dash goes out - in under 1.5 miles. In winter 2-2.5 miles.
No much difference in noise as it's CR and does not make much start up clatter (one of the reasons I bought it as I hate noise)...
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The economy on cold is an easy one. Diesel engines do not "run rich" (i.e. run on choke) on cold, whereas a petrol engine does. A cold diesel engine will typically advance its injection timing, and possibly step the idle speed up slightly, but the actual quantity of fuel injected remains the same regardless of temperature. A petrol engine on choke burns significantly more fuel than one at operating temperature.
The extra noise of a cold diesel relates to compression. Diesel engines typically run compression ratios twice that of a petrol engine. The heat from this ultra high compression is what ignites the fuel as it is injected into the cylinders. A cold engine has lower compression than a hot one, due to the piston to cylinder clearance being larger. This, combined with further loss of heat into the cold cylinder walls affects the combustion process, with the result that the diesel "knock" is more pronounced.
Long warmup time is due to a diesel engine being much more thermally efficient than a petrol, so less waste heat being transferred into the cooling system. This is the biggest problem with the diesel engine in my experience. The warmup time on our Scenic compared to the Volvo is about four times longer, and if you get caught in traffic, the engine doesn't actually warm up at all.
Edited by DP on 29/09/2008 at 14:51
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I believe (from my physics@school days) that colder materials (esp. metals) conduct & propapgate sound more easily & readily compared to the same when warm/hot - so perhaps a factor in the overall higher sounds levels from a cold engine.
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Had to nip out a while ago. Took my diesel Mondeo which hasn't moved for a few days. Prompted by this thread I turned the heating on to check how long it took to come through. About a mile into the journey it was blowing hot air. Must be something wrong with it.........
;-)
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it was blowing hot air. Must be something wrong
Have you perhaps politicised it inadvertently, HB?
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Heh heh ! It did at one time pull to the left but I had that fixed.
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Don't some diesels have electric immersion water heaters to give some warm air a bit earlier?
Bizarre, really, coolant warming the engine rather than the other way around.
Edited by ifithelps on 29/09/2008 at 21:01
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The proper reason why they are noisey cold is the fact that as they don't run fuel enrichment cold ( as pointed out by DP) the diesel injection timing is advanced to compensate for a slower burn. Hence this is where the knock comes from is the advanced combustion. As diesel burns slighly slower the timing has to be well advanced.
On the whole the average diesel engine is built in a much stronger & heavier material & often have a larger water capacity & the combustion tempretures tend to be lower hence the longer warming up period.
The only reason economey would improve hot is due to the fact that the engine produces more power due to effienct combustion & sutible timing hot than cold. They do not burn signifantly more fuel cold but need a heavier right foot.
Part of the diesel ( cack noise ) is created by the difference in tempretures of fuel being injected into the engine.
I Doc
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