Thread: 300tdi Injection Pump - Fuelling Understanding?

You are correct in that the Gas laws govern the output of an internal combustion engine, and that density and volume of incoming air (Cooler = denser) quality of fuel/ compression ratios(rate of burn etc) and overall efficiency of the rotating parts within the engine itself all change due to outside influences such as ambient temps and pressures. Usually unless it is quite a big ambient temp differential you would have trouble picking up the actual change in performance.

JC

Thanks JC, was hoping you might chime in

Sub 10 degrees cool moist air.
VS
Say 35 dry air temp with at a guess 45 degrees air at IC level.

enough? or is it purely subjective and maybe I just prefer dodging cattle and wobbalies in the dark and hence feel things are "better".

I am going to put an EGT gauge in for interest sake, is there some practical form of O2 sensor that you could put in the exhaust tract to see changes in combustion? Or maybe I need to get two thermocouples and run one in the inlet manifold and then can switch between EGT and Inlet Temp... would be interesting to know the differences but largely irrelevant I imagine?

S

Originally Posted by roverrescue

I think this is one for the experts.

Driving back home from Cairns last night - it was chilly for a North Queenslander (sub 10 degrees I presume) and the 130 was on song all the 330km home. I noticed a distinct power improvement over the run down in the middle of the day on Saturday (Hot and dry out between lakeland and Molloy). In the cold the 130 was happy sitting at 110kph and even had more to go when at a similar throtte position on saturday where it was only happy doing 100kph.

Anyway I got thinking about how the 300 Injection Pump is set up.

The turbo has essentially a fixed boost pressure at a set RPM.
According to Boyles Gas Law to maintain that pressure the two variables are Volume and Temperature... so as charge inlet temperature goes down (chilly night so airbox temp is down and post intercooler inlet temperature is also down) then the volume of air (O2) has been increased. All makes sense so far.

Now the 300 IP has a boost sensor, so I can understand that changes in inlet pressure are compensated for but there is no volume (MAF / O2) sensor so I can only presume that fuelling is essentially set for a predetermined air charge volume at a certain boost pressure... The volume is related to temperature which means the pump setting is "optimized" for a specific ambient air temperature. Am I on the right track. The optimized temperature assuming it is a UK design would be closer to Sunday night than Saturday lunch maybe???

Is this the reason why my fuel consumption seems to go up when its FNQ hot (talking 13L/100) compared to when its a night run 11L/100. Effectively when its hot the engine is under volumed or conversely over fuelled?

I tried to do the maths while driving; at 2800RPM the 300tdi should need 58L of air per second. Is the little hairdryer capable of pumping that volume when its hot (like 35 ambient 50-60 road temp) at the factory pressure.

Now Im sure Im missing some vital clue, (like a dieso isnt actually anything but an airpump and there is always more air volume than required for the stoichiometric ratio - I want numbers ) but I want to know if Im thinking right?

I also figured the easiest way to rectify this situation (if it is a situation) would be to adjust the boost upwards in high ambient temps.

Any takers?

Steve

Diesels don't run at stoichiometric ratio. They run much more air. From memory 18:1 is the approximate smoke limit for a diesel - stoich is about 14:1.

Another parameter that you haven't factored in is the atmospheric pressure.

Load on engine affects how much boost the turbo makes. They are not a fixed boost pressure at a set RPM. See how much/little boost pressure you can get by increasing engine revs while vehicle is stationary.

Steve - I would reccommend fitting an EGT ASAP. As I mentioned previously, the tests of your Provent so far suggest that soot levels in your oil may be high - don't forget to send me that oil sample when you have a moment...

Originally Posted by isuzurover

Steve - I would reccommend fitting an EGT ASAP. As I mentioned previously, the tests of your Provent so far suggest that soot levels in your oil may be high - don't forget to send me that oil sample when you have a moment...

Do you have any details of the other Provent results?

Ben,
thanks mate... sorry have been crazy busy moving house / jobs and what not...
defender is a few kays overdue for a service, will get onto it this week and get a sample your way.

I have been wondering if my EGT are too high as I have found the following:

new 4 core 4 pass radiator
new thermofan clutch
new coolant
new waterpump
VDO temp gauge and probe

engine will cruise nicely at say 90-95 degrees celsius but then as soon as you get to a hill it way too easily starts to climb up to say 110 degrees max. It just seems way to much to be related to fuel input (ie EGT soaring) which is heating the head.

probably need a new thread but... the only IP mod I have is bumping up the idle screw a half turn??? What can cause increased EGTs besides increased fuel...

maybe just maybe could my turbo be underboosting or some such????





Back to topic thanks John I though diesos were above stoich so that makes sense. Atm pressure didnt change much, I got the following from the BOM

DAY RUN - air temp 23.2
rel humidity 43%
atmo 1017HPa

NIGHT RUN- air temp 16.1
rel humidity 85%
atmo 1021HPa


so only 7 degrees temp difference however I imagine at road surface in the day the temp would be much higher??? but the doubling in humidity is interesting...

Much thinking/ reading measuring to be done!

S

IMHO I think your temps are up but it depends on humidity and temp.My Tdi runs right on the thremo temp at mid twentys and normal driving,around 85-90 degree's,it only runs higher in summer when the temp really gets serious,mid 40's,then it see's 100 quite easily.If I was to use it for towing in summer I would do some serious venting work in the engine bay. Pat