Which way is better to diesel engine ? ...........

Submitted: Sunday, Sep 28, 2003 at 12:41
ThreadID: 7482 Views:1714 Replies:7 FollowUps:3
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From my previous message posted, i know Engine run best after warm up...............
However, during cold start..........which way is better to diesel engine?

1. Long idling for 10-15mins so that to keep the rev as low as possible (prevent high rpm before warmed up), .......


2. Rev it to about 1500-2000rpm for few mintues to quickly warm it up (but high rpm when engine still cold)........

I have been thinking of this questions for long time. Can anyone tell me the answer? THANKS...........
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Reply By: David N. - Sunday, Sep 28, 2003 at 14:15

Sunday, Sep 28, 2003 at 14:15
IMHO- many years experience, +an engineering degree, + and rebuilt more than a few engines for mates... and girlfriends over the years.

1. Avoid cold starts as much as possible (ie: combine your trips where possible)

2. When you must cold start, drive gently as soon as the oil pressure has come up. Don't rev much, don't labour the donk, don't accelerate hard. The operative word is GENTLE when it's cold. Long idle is not effective especially on a diesel as it will take forever to warm up. Then if you must thrash your engine NEVER do it before it's fully warmed up.
AnswerID: 32229

Follow Up By: IAMGQ - Sunday, Sep 28, 2003 at 14:40

Sunday, Sep 28, 2003 at 14:40
Thank you David.
Any patricular reason why diesel engine take ages to warm up if just idling.......?
So from what you said, it is better to warm it up by drive it gently rather than sit there and idle ,right ?

FollowupID: 23016

Follow Up By: David N. - Monday, Sep 29, 2003 at 09:54

Monday, Sep 29, 2003 at 09:54
A diesel at idle (or virtually at any revs for that matter) is pumping much more cold air through than a petrol. (a petrol motor has a throttle butterfly) Therefore on a diesel at idle, with a very small amount of fuel going thru, there is not much heat being "created".
As an example, watch any petrol engine with "thermatic" ie: electric fans- the motor warms up quickly- even at idle, & the fans then cycle off and on regularly as the engine is making plenty of heat- even at idle. (petrol engines use far more fuel at idle than diesel + less air going thru = more heat created.)
Watch a diesel with electric cooling fans and you might watch it for a long time indeed to see the electric fans cycle off and on.
My father's diesel Peugeot for example, could probably idle all day in cool weather without the electric fans cutting in- without overheating. Obviously when the engine is working harder it is a totally different ball-game.
FollowupID: 23061

Reply By: ross - Sunday, Sep 28, 2003 at 16:57

Sunday, Sep 28, 2003 at 16:57
Ok as I mentioned earlier 1 minute for each litre so 4 ltr= 4min.The reason I am convinced that a diesel must be warmed slowly through idling is a infra red photo I was shown of a petrol and diesel motor shot side by side .After about 5mins the petrol motor was warmed up enough for the infra red photo to show warmth all over the block and heads,while the diesel of a similar capacity had hot spots localised around the combustion chamber. This is because a diesel is built out of much heavier casting than a petrol motor to cope with a compression ratio of about 22:1 which is approximately twice that of a petrol motor.The heavier casting takes much longer to warm up and expand and the diesel is also much hotter internally so therefor it needs to be expanded at a slower rate to stop pistons/rings being squeezed against the cyl walls. The older the design and bigger the capacity of the diesel,the more critical a slow warm up is.Modern diesels are made of lighter,more durable metals and can warm up quicker but still not as quick as petrol motors.
AnswerID: 32232

Reply By: jeepy - Sunday, Sep 28, 2003 at 20:01

Sunday, Sep 28, 2003 at 20:01
Best thing to do is just drive gently until the engine warms up. A lot of people forget that its not just the engine that has to warm up, its also the gearbox, diffs etc.
This guy who used to live in my street would religously idle his car for about 15mins before driving off and then nail the car after that and he couldn't work out why he was going through diffs so fast.
Idling just creates pollution, wastes your fuel and time and means that the engine is running at a colder temp for longer than necessary. Exactly the same principles apply for diesels and petrols, its just you may have to drive the diesel gently for longer.

AnswerID: 32240

Follow Up By: Member - Bob - Sunday, Sep 28, 2003 at 20:21

Sunday, Sep 28, 2003 at 20:21
I agree with that. Don't idle it. Drive without excessive revs or acceleration. Friction is related to speed and high engine revs means the pistons are travelling very fast indeed. So gentle throttle use (minimise turbo boost), and slow engine speeds will ensure the engine warms as quickly and as evenly as possible without too much loss of metal from moving parts.Bob
FollowupID: 23034

Reply By: Martin - Sunday, Sep 28, 2003 at 20:36

Sunday, Sep 28, 2003 at 20:36
IAMGQ ... I agree that it's best to warm up by driving gently as this warms everything up - engine and all transmission parts. One other factor that makes diesels slower to warm is their larger oil capacity - often nearly double that of a comparable petrol motor. Don't let it idle for long periods in the morning - drives the neighbours mad and it's not necessary.
AnswerID: 32247

Reply By: Martyn (WA) - Sunday, Sep 28, 2003 at 21:50

Sunday, Sep 28, 2003 at 21:50
I agree with the short idle and then "gently bently" for a short period to get everything up and moving around, personally I wait for the oil pressure gauge to get the oil where it needs to be, usually about 30 secs and then I drive gentlt until the needle moves on the temp gauge before I put my hoof into it at all. I have a thermo couple in the exhaust so I can monitor the temp easier, be cause I have a turbo. If you leave the engine to idle for 15 mins the thermo couple doesn't move off 100 degrees C for ages so the theory mentioned above about not heating much up and taking a long time bares a lot or credance.
Something else, when starting the engine as you are taught you depress the clutch before you start the engine and release after the engine has fired, just in case the gearbox is in gear, in my opinion this is one of the biggest engine killers, you are loading up the crankshaft thrust shells with virtually no lubrication at all when you turn the crank for the first time, I always now make sure the gearbox is in neutral and then start the engine without depressing the clutch. If you want to test this theory try it out, you will find that the engine will spin slightly faster without the clutch depressed. This is also a good tip when the battery isn't in the best condition, the engine requires less "juice" to turn it over.
Just a small thing, hope it helps Keep the shiny side up
AnswerID: 32261

Reply By: Member - DOZER- Monday, Sep 29, 2003 at 10:36

Monday, Sep 29, 2003 at 10:36
Diesels are more thermally efficient than petrols. Direct injected diesels are more thermally efficient than indirect injected diesels. Thats why they use less fuel, because more portion of the explosion is turned into power, and less into heat. Ontop of this direct injection has the combustion chamber in the piston, where heat doesnt escape to the cooling system as much, so more efficiency there aswell.
Once the colant is warm, dont think that the oil is also - it takes longer to heat up and thin out, so, after start up, idle for a period of time dependant on ambient, (colder equals more) Gentle as she goes for a few minutes, (especially with turbo diesels) and then work it as hard as you like!
Stay away from the 1500/2000 after start idea.... remember as a kid the neighbour did that to his Kombi motorsssssss :-)
wheredayathinkwer mike?
AnswerID: 32285

Reply By: Truckster (Vic) - Monday, Sep 29, 2003 at 11:24

Monday, Sep 29, 2003 at 11:24
I usually warm up the engine start it, leave idling for a min, then on hand throttle for 2 mins at around ~1000rpm

then drive away slowly, as others have said, everything needs to warm up.
AnswerID: 32292

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