A little question about diesels

Submitted: Friday, Aug 22, 2003 at 08:34
ThreadID: 6748 Views:2346 Replies:10 FollowUps:9
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A little question for the technically minded: Why do diesels utilising pre combustion chamber, ie indirect injection, generate more soot in the oil (thickening) than direct injection over the piston top? What is the purpose of the pre combustion chamber? Is it to slow down the combustion of the fuel? If so would advancing the injection pump reduce oil thickening? Some engines (2h) have static advance of 18deg. whilst 1hz is tdc. Both indirect injection. Tdi has direct injection and barely discolors the oil in 5000k. As does our D7 in 150hrs.
Some help please
cheers haze
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Reply By: Member - Bob L - Friday, Aug 22, 2003 at 11:55

Friday, Aug 22, 2003 at 11:55
Ive no idea, But I do believe the Tdi has a centrafugal oil cleaner which removes the carbon allowing greater time between oil changes (20,000km ?)
Cheers Bob

AnswerID: 28719

Follow Up By: haze - Friday, Aug 22, 2003 at 13:13

Friday, Aug 22, 2003 at 13:13
Bob . the 300tdi just uses a ryco z89a filter, a fairly common type used on several other vehicles. And no mention of centrifugal filter in L/R w/shop manual. It does have a thermostat controlled oil cooler however.
Book says change oil 10k, filter 20k. I do both at 5k.
cheers haze
FollowupID: 19978

Reply By: Hughesy - Friday, Aug 22, 2003 at 13:24

Friday, Aug 22, 2003 at 13:24
The reason they generate more soot is because they are a less efficient motor (at burning the available fuel) and hence contaminate the oil quicker, with unburnt fuel and the by-products of incomplete combustion. No engine burns up all the fuel completely, some just do a better job than others ie. direct injection motors. The higher the compression ratio of the motor generally the more efficient it will be and that's why most new high tech diesels are DI. That is also why an indirect injection motor relies so heavily on its glow plugs for cold starts because the compression ratio is lower than that of direct injection motors. The reason why we have Indirect injection is a little beyond me, but they are a quiter motor because the combustion phase is spread over a longer time period and hence no loud "knock" or rattle like a direct injection. The other thing that will soot the oil up is how hard the engine is being worked. If the motor is working hard and at near max revs (like your D7 should be!!) then this is where the efficiency of the motor is at its best and hence will burn cleaner than a diesel being putted around town in the old ute. Diesels love to be worked hard so that all the old bleep e that builds up can be burnt off.

But generally DI is the way to go and correct me if I'm wrong readers but I think you'd be hard pressed to find too many new diesel model vehicles that are indirect injection other than the old 1HZ, in the cruisers. Anyway better stop now, hope this helps Haze.

AnswerID: 28729

Follow Up By: haze - Friday, Aug 22, 2003 at 13:58

Friday, Aug 22, 2003 at 13:58
Thanks, Hughesy I am now somewhat enlightened but for one thing- comp. ratio. The Tdi is 19.5:1 1hz 22.7:1 1hd-t 18.6:1, 2.25l. landie diesel 23:1 (From manuals) showing the indirect injection engines quite higher C/R
And agree about being worked hard, and they all get that. Long runs and heavy load! As for the 7, I wish it was a 9!! But then up would go the fuel bill without any bigger guarantee of return.
Thanks for your thoughts on the subject
cheers haze
FollowupID: 19982

Follow Up By: Patrollin - Friday, Aug 22, 2003 at 16:01

Friday, Aug 22, 2003 at 16:01
Hughesy, is that why holden rodeo TD are such noisy rattly buggers? Direct injected?
FollowupID: 19996

Follow Up By: Old Jack - Sunday, Aug 24, 2003 at 16:18

Sunday, Aug 24, 2003 at 16:18
Just remember that the TDI rover, Jackaroo- rodeo motors are turbo charged and once on boost they have a much higher effective compression ratio and in the Jackaroo and other electronic injected motors far more precise injection control of fuel. also the common rail injection pressures are far higher than mechanical injection pressure wich result in better fuel atomisation giving cleaner combustion.

and yes the rodeo's are direct injection. one day they will put the CAT injection off the jackaroo on then and zip the output up to 118kw!
FollowupID: 20144

Reply By: Member - Russell - Friday, Aug 22, 2003 at 15:30

Friday, Aug 22, 2003 at 15:30
I'm no expert either, but my understanding is that common rail system is better again, because it is not limited by a mechanical fuel pump. There's only so much control you can get with a mechanical fuel pump, regardless of whether it's direct or indirect injection. In contrast, the high pressure rail allows the computer to put exactly what it wants in the combustion chamber at precisely when the cycle needs it. I've also heard that some can inject at multiple points in each combustion cycle. Hence more power and more complete burning - and hence cleaner oil..... Please correct me if I'm full of ####.....Russell S
Prado RV6
AnswerID: 28749

Reply By: GoRodeoGo - Friday, Aug 22, 2003 at 16:38

Friday, Aug 22, 2003 at 16:38
In short, here's the low down for Direct Injection and Indirect Injection.


1. As all the air is compressed within the cylinder and all combustion occours therein, the heat losses are not great and a high thermal efficiency is obtained
2. Due to low heat losses the engine can be started from cold without the necessity of heater plugs
3. Good fuel economy is obtained due to the high thermal efficiency

1. Rough running due to high cylinder pressures caused by delay fuel accumulating in main cylinder
2. Ignition value of fuel is very important as this affects delay period
3. Relatively small nozzle orifices which are more prone to blockage
4. Higher injection pressures cause more rapid wear of injection equipment


1. Poor quailty fuels can be burned as the high pressure at the end of the delay period does not operate on the piston
2. The engine runs very smoothly due to the low maximum cylinder pressure.
3. High compression ratios to 22:1 can be used without excessively high cylinder combustion pressures.
4. Low injection pressure used, also pintle type nozzles which reduce injection system maintenance

1. Considerable combustion occours away from the cylinder therefore a lower thermal efficiency is obtained and poor fuel economy results
2. Glow plugs must be used in order to facilitate cold starting
3. High compression ratios must be used to compensate to heat loss

AnswerID: 28759

Follow Up By: StephenF - Saturday, Aug 23, 2003 at 12:00

Saturday, Aug 23, 2003 at 12:00
Good summary Stuart, although I assume the "poor fuel economy" of the indirect injection is when compared to the direct injection - it's still a hell of a lot better than petrol!

In the past indirect injection was used in car-type diesels (Mercedes, Peugeot, VW) because it was quieter and smoother, and direct injection was usually found on larger truck/tractor engines where noise and smoothness didn't matter. However computer-controlled common-rail direct-injection diesels have become smoother and quieter and have probably signalled the end of indirect injection.

(1981 Peugeot indirect injection diesel)
FollowupID: 20077

Reply By: Member - DOZER- Friday, Aug 22, 2003 at 19:53

Friday, Aug 22, 2003 at 19:53
Some motors are ment to run dirty (like jap non direct diesels) they have a large filter to catch the crud and a 5000km change, others like the American direct inj motors have small filters and a service interval of 10-30000kms.
The precombustion chamber is solely there to mix air and fuel so it can be ignited.
Direct injection relies on smaller injecters to atomise better and the compression heat ignites the charge.
Go the D7
Andrewwheredayathinkwer mike?
AnswerID: 28775

Reply By: haze - Friday, Aug 22, 2003 at 21:07

Friday, Aug 22, 2003 at 21:07
Now you blokes have really given me some stuff to get my head around! So ok, a lower C/R direct injection doesnt need glow plugs to fire, but a higher C/R indirect does. But I have to think about that little slot that leads into the pre comb. chamber and relate it to the output from my little air compressor ie: where the compressed airflow is restricted it gets as hot as all getout! Why isnt that hot enough to light the wick? - obviously not. And when it does (finally) ignite it dont want to burn completely anyway. It does seem sensible to whack the fuel in where its needed,smack on top of the piston, so have the "indirect injection" proponents been selling us a dud? (always been happy with the chatter of the tdi, now thinking about the purr of the 1hz!) As to the D7G, it has a fuel pump which primes the injection system which in turn is actuated by the camshaft. A governer controls injection rate. Not sure if this is "common rail" but certainly no electronics involved. Now 8500+ hrs. and never touched. Uses 1-3l. oil in 150hrs. Yeah! go the 7!!
I now feel a need for some deep reseach (plus a rum or 3)
Once again, thanks, haze
AnswerID: 28784

Reply By: Tony - Friday, Aug 22, 2003 at 21:10

Friday, Aug 22, 2003 at 21:10
Has this got anything to do with 'Explor Oz'...or are you using this site as a forum for self promotion ? Why don't you contact the fuel companies if u are fair dinkum ??
AnswerID: 28785

Follow Up By: haze - Friday, Aug 22, 2003 at 21:34

Friday, Aug 22, 2003 at 21:34
Tony, dear sir. I asked this question because I happen to own a number of different types and makes of diesels. So, what has this to do with Explor Oz, I dunno. But does that make it out of bounds? Is asking about what filter, oil should I use out of bounds? Is talking about what people for a living anything to do with Explor Oz? Get a grip tony (if there is anything to get hold of) And when did fuel companies have anything to do with engine design? Better you stay on the medication
FollowupID: 20036

Reply By: Fred - Friday, Aug 22, 2003 at 23:38

Friday, Aug 22, 2003 at 23:38
Bit touchy hazey????
AnswerID: 28804

Follow Up By: haze - Saturday, Aug 23, 2003 at 07:27

Saturday, Aug 23, 2003 at 07:27
Yes, Fred, I spose you could say that. But you know some dudes have given up learning by the time their 20, others never stop. I want to belong to the latter group! And am pleased that out there are those happy to pass their knowledge/ideas along
FollowupID: 20052

Follow Up By: Waynepd (NSW) - Saturday, Aug 23, 2003 at 20:51

Saturday, Aug 23, 2003 at 20:51
I'm with you Hazey,
I am old enough to remember the beatles' early days and still smart enough to learn new things. Stop learning and you might as well be dead.
I got a lot out your post and the replies to it. I have wondered about the different formats of diesel injection but didn't know how to ask it.

My GU Patrol is the non-turbo I/D 4.2 diesel and the oil is black at the 5000km change. Averaging about 12L/100Km. No powerhouse but adequate for my needs.

FollowupID: 20103

Reply By: haze - Sunday, Aug 24, 2003 at 15:03

Sunday, Aug 24, 2003 at 15:03
Waynepd. Bought the 1hz new so no prior history to consider. Its a workhorse and often (ie Very often) is towing heavy loads - like 8 drums of fuel in a tandem trailer. And seemingly quite typical of toyota, plenty of black smoke following behind. I wondered what might happen if I turned the injector pump back a whisker, and so the result its possibly down a bit on power, the smoke is all but gone (can still be seen from the lights of a following vehicle) but best of all the oil is nowhere near as dirty at 5k. as previous. No noticable change in economy, around 14l./100km. towing, but speeds are down probably 10%. What the hell! we still get there.
cheers haze
AnswerID: 28909

Follow Up By: Old Jack - Sunday, Aug 24, 2003 at 16:25

Sunday, Aug 24, 2003 at 16:25
with regard to the 1hz injection has the timing been checked? they tend to smoke worse if the injection timiing is out of whack( most deisel's do)
Happy motoring!
FollowupID: 20145

Reply By: haze - Sunday, Aug 24, 2003 at 18:12

Sunday, Aug 24, 2003 at 18:12
Thanks, old jack. Good one. I note the spec. is tdc. I checked with a home made pipe to fit no.1 injector and to get what I reckoned was injection start I had to advance the pump 2deg. Does this make sense? Does it smoke with more advance or retarded. I have a feeling these jap motors get their claimed HP (kw.) from a fair bit of over fuelling? Thanks
cheers haze
AnswerID: 28927

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