Monday, Sep 12, 2005 at 11:32
Hi Guys,
Hmmm, not sure what 'disel bread' would taste like but, as Ken (Billowaggi) has invited me to comment, here's the guts of my recent reply to Andrew's post no. 26274:
"Hi Andrew,
Wish I'd spotted your post earlier before so many 'diesel myths' had surfaced again...
I'll try to dispel a common myth - it is impossible to run a diesel 'too lean'. They always run with air/fuel ratios on the 'lean' side of stoichiometric. At idle, the ratio is hugely 'lean' - just a tiny squirt of fuel is injected into a cylinder full of air. At full throttle, the ratio is closer to stoichiometric but will never actually get there without generating damagingly high exhaust gas temperatures (EGT). So it is actually 'rich' ratios (overfuelling)that cause trouble in diesels...
Please note, I've used the term 'ratio' rather than 'mixture' as diesels never actually have an air/fuel 'mixture' in the way petrol engines do. The only time a 'mixture' exists is in the instant between the beginning of injection and the beginning of combustion - an matter of milliseconds at most. Enough diesel theory - back to the subject at hand...
Non-factory-turboed engines have injection pumps without a manifold pressure compensator (otherwise know as a boost compensator or, somewhat erroneously, as an 'aneroid'). Putting a turbo onto a 'normally-asphyxiated' diesel with a non-compensated injection pump will always be a compromise. Without a manifold pressure compensator, the pump can never deliver the optimum quantity of fuel in both off/low-boost and full boost conditions. BUT, the results can still be quite acceptable and reliable, as long as the engine is not heavily over-fuelled at full load...
...After fitting the turbo kit, this (maximum fuel delivery) will need to be adjusted to increase the full load fuelling in line with the increased air charge. It is critical that this be done while monitoring EGT, either with a temporary gauge or after installing a permanent gauge kit.
In Andrew's situation (non-compensated pump with moderate boost) it is generally possible to find a maximum delivery adjustment setting which gives satisfactory performance without excessive EGT (or smoke) at full boost AND not too much smoke (overfuelling) during low boost acceleration.
If you are confident with your mechanical abilities, I'd recommend you fit an EGT gauge and monitor your engine under 'real world' conditions. [Yes, I do sell them but I really believe any non-standard turbo diesel should have one. If you don't like
mine, get another brand - but do get one!]
If you find your maximum EGT is
well within acceptable limits when under heavy load at full throttle (say, less than 650 C upstream/450 C downstream), you probably have some scope to increase the maximum delivery adjustment. Conversely, if your EGT is reaching high levels (700+ upstream/500+ downstream) within a short period at high load, your max. fuel delivery may already be a bit too high.
But also remember, pump delivery adjustment is only part of the story. The entire fuel system needs to be in good order to get reliable performance. General maintenance items like valve clearance adjustment needs to be correct. If the engine already has high kms (say, >200,000), it's probably advisable to have the pump and injectors checked and re-calibrated/reconditioned/replaced by a reputable specialist. Also the injection timing needs to be spot-on. This is another area where the accumulated experience of specialists like Denco comes in. Any competent 'home mechanic' should be able to check the timing to factory spec but experienced specialists will know whether non-standard timing is required on different engines after adding a turbo, different boost levels, with or without EGR, etc.
Hope this helps."
Again I hope this clarifies these particular diesel issues.
Ian
FollowupID:
384124