direct/indirect diesel injection

Honest John Back Room » The Back Room Archive
What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks
View Threaded
Thu 29 Mar 2001 18:30
direct/indirect diesel injection - Ashley
Hi all,

I have purchased a 'w' plate VW Polo 1.9 SDi, it has the new direct injection engine fitted. My current Polo is indirect injected and uses glow plugs to start. I know the fundamentals of indirect injection, but how does a direct injected car start like a petrol, i.e straight away without waiting for the fuel to be pre-heated ?
Does the direct injection use glow plugs, and if not is the fuel heated up by the action of the piston moving up the cylinder and compressing the mixture ?
I am collecting the car next week so i have not had a good look at the engine yet so any info would be appreciated.

Regards,

Ash.
Reply | Report as offensive | Link
Thu 29 Mar 2001 19:29
Re: direct/indirect diesel injection - steve paterson
A DI. engine has a higher compression ratio than an indirect engine. Typically about 25:1 as opposed to about 18:1 for indirect. Also the fuel is injected directly into the combustion chamber, not a precombustion chamber.
The heat generated by the compression of a DI engine is enough to start it without using glow plugs etc.
Reply | Report as offensive | Link
Thu 29 Mar 2001 21:44
Re: direct/indirect diesel injection - Alvin Booth
Ashley,
All diesels both DI and IDI work on the same principle of the combustion taking place by compression alone which generates the heat required to fire the diesel which is sprayed in by the injectors at high pressure.
As Steve says the only difference between an indirect and direct injection is that with the indirect there is a very small chamber above the normal combustion area above the pistons.
The indirct was designed to reduce diesel noise, it is less efficent than the DI because of more heat loss and less efficient but at the time it made the diesel engine acceptable in areas of refinement with Peugot at the forefront.
Probably the first DI engine fitted in cars although this is arguable was the Perkins Prima developed in conjunction with Rover which was fittted to the Maestro and Montego.
Since then all manufacturers have gone to DI and technology has advanced using various means to soften the combustion noise including two stage injectors. The DI does have glow plugs exactly the same as your IDI but only needs them in very cold weather for the first start and then only for a couple of seconds.
In my vectra for instance the glow plugs only operate when the outside temperature is below 3C. In actual practise you can forget about any pre-heat and simply start as you would with a petrol car

regards

Alvin
Reply | Report as offensive | Link
Fri 30 Mar 2001 18:21
Re: direct/indirect diesel injection - Ashley
Thanks for taking the time to reply gents, it's much appreciated. I'm now looking forward to collecting it !

Ash.
Reply | Report as offensive | Link
Sat 31 Mar 2001 22:25
Re: direct/indirect diesel injection - Andrew Hamilton
Perkins engine also was fitted to the LDV 200 panel van. Fixed heating time on every start whatever the outside temp.
Reply | Report as offensive | Link

If you cover no more than 100 miles per day and have easy access to a charging point at home or work, then an EV is ideal for you.

Is an electric car right for me?