Cold start fuel ecconomy. E300 td.

[HERBIEMERCMAN]

somone once told me that one of the advantages of a diesel engine was they did not suffer short runs like a petrol engine on cold start choke ? recently my car has been on short runs, basically driven out of a garage in the morning and then put back in the evening. this happened for seven days waiting for spare parts. when i filled up with fuel i calculated 19 mpg, the lowest figure i have ever had in ten years. usually the lowest in winter is 26mpg short runs and 35 mpg long journeys. so the honest but wrong beleif on a diesel is not correct. come to think of it i have often heard the engine rpm drop after 5 mins, this must have been when the cold start overfuelling was switching over. they must set the cars for the worst case scanerio for german winters. you will never see a milkman with a merc ! herbiemercman.

 

[turbopete]

cant say ive noticed mine run faster on start up. always idles around 600rpm. heater plugs sta on a while after cold starts to help fuel economy, but id guess that figure is about right as its similar to what i get around town going to work etc, ie trips of around 2miles or less at the moment!

 

[Number_Cruncher]

There is no cold start overfuelling on a diesel.

If more diesel is injected, the engine produces more output, and so if in neutral would rev up. Varying the quantity of fuel injected is the only way that a diesel's output is regulated - there's no control over the air intake.

This is in contrast to a petrol which can only draw in a small amount of air past the closed throttle, and if more fuel is injested under these conditions, the change in engine output is minimal.

On some diesels, there is a cold running advance mechanism to inject the fuel earlier, but that feature isn't built into the inline pumps on the OM606.

 

[dougjoy]

So the deisel knock in cold weather is not down to a richer mixture of oil and air as I had assumed???

 

[roadhog]

I thought the diesel knock when cold was down to a more inefficient combustion while the engine isn't at operating temperature?

 

[HERBIEMERCMAN]

i have a boat with two ford 590 e diesel engines, on the side of the simms pumps there is a cold start button which provides more fuel on cold starts. if this button is not pressed the engines will not start. i also contacted my friend who is the manager of "merlin" diesel in preston, they have a reputation for their expertise, they told me that my car increases fuelling for cold starts, and i can hear the operation of the engine distinctly change after 3 or 4 minutes from cold. i am confused with numbercruncher's comments, which to my interpretation is saying that the diesel engine on my examples do not have a cold start facility ? the fuelling is the same hot or cold, so what are the cold start buttons on the pump doing ? why does my merc. engine change, noticeably after a cold start ? why are "merlin" experts saying what they are saying ? herbiemercman.

 

[television]

All engines need more fuel from a cold start

 

[Number_Cruncher]

At cranking speeds, the fuel rack on these pumps is automatically moved towards maximum fuel, and as soon as the engine catches, the fuel rack position is controlled via the governor - on W210 OM606s, this is via the engine controller, and W124 OM606s via the ELR system.

The same is trus of old engines with excess fuel devices as mentioned higher in the thread - they are purely for starting - as soon as the engine is running, the control is passed back to the governor.

The change in engine note you hear is the afterglow being turned off, unloading the alternator.

As soon as they are running, diesels need no enrichment - there's no facility for it. To overcome the extra friction in the cold engine oil, and to offset the poorer combustion, the fuel rack will naturally be moved by the governor towards maximum fuel to obtain the correct idle speed, but, this is a continuous correction.

Unlike a petrol, the output of a diesel is purely controlled by how rich the mixture is - the air is not throttled or controlled. Add more fuel to an idling diesel engine, and it will rev up. Add more fuel to an idling petrol engine, and you don't need to add much before it runs rough and stalls because the mixture is too rich to burn. The method of control between the 2 types of engine is completely different (despite how similar a common rail diesel system looks to a petrol injection system)