Thread: 4BD2T Inection Pump on a 4BD1T?

Originally Posted by Bush65

.... Combustion of the fuel starts in the pre-combustion chamber and expands into the cylinder as the piston descends. No atomised fuel finds its way into the cylinder when the engine is running normally.

I should have added that the fuel that is ejected from the orifices of the pre-combustion chamber, into the cylinder is vapour.

Compared to direct injection, indirect injection diesels are capable of much higher engine rpm's, and higher power as a result.

The reason being the time taken for the diesel molecule to be broken and be vapourised so it can combusted as I mentioned in an earlier post. As the piston descends on the expansion/power stroke, the temperature and pressure drops quickly shortly after TDC.

In a direct injection diesel this expansion, resulting in reduced temp and pressure, slows down the rate at which the diesel molecules break down to where combustion can begin. This is another reason why injection has to finish so soon after TDC.

In the indirect injection engine, the orifices in the pre-combustion chambers reduce the pressure drop in the chamber giving more time, or same time at higher rpm for this process. The difference in pressure between cylinder and precup is considerable.

The downside of indirect injection is less efficient (higher fuel consumption for same power, ~15% less than DI) and less torque at low speed. However they are quieter and have a broad power band like a petrol engine.

I've tracked down the injection timing specification for the two pumps:

4BD1T - 13 degrees BTDC
4BD2T - 8 degrees BTDC

On a 50 tooth pump gear, a single tooth has a 7.2 degree range, or rather, an adjustment of +/- 3.6 degrees from the mean position. Considering the difference of timing specification of 5 degrees, the timing of the 2T pump on a 1T should therefore be one tooth off the alignment mark. If I've got that right, the pump gear alignment mark should be over-rotated clockwise by one tooth (i.e. looking at the front of the engine) from the idler pulley alignment mark.

Is that right?

not quite, if you step over a tooth you're at 7.2 degrees from the mean position for the pump and thats 14.4 on the crank.

what you need to watch is how much slop there is in the fine timing adjustment if you need to add 5 degrees of advance but only get +/- 2 degree on the pump (+/-4 on the crank) you cant hit the optimal timing point just by advancing the pump one tooth then winding the fine timing all the way back. (assuming you start at TDC stepping one tooth advanced gets you 14.4BTDC then rocking the pump all the way back gets you back to 10.4BTDC)

Originally Posted by Blknight.aus

not quite, if you step over a tooth you're at 7.2 degrees from the mean position for the pump and thats 14.4 on the crank.

what you need to watch is how much slop there is in the fine timing adjustment if you need to add 5 degrees of advance but only get +/- 2 degree on the pump (+/-4 on the crank) you cant hit the optimal timing point just by advancing the pump one tooth then winding the fine timing all the way back. (assuming you start at TDC stepping one tooth advanced gets you 14.4BTDC then rocking the pump all the way back gets you back to 10.4BTDC)

Thanks Dave, I forgot about the crank : pump gear ratio... WRT fine adjustment, I assumed rotating the pump on the pump flange would give you at least +/- 3.6 degrees range on the pump gear, so that the timing is fully adjustable... (that's how I'd design it in any case)... will need to check before throwing it on...

The good news is the timing marks will most likely align as a 5 degree crank angle is only 2.5 degrees at the pump.

Thats making a few assumptions.....

the 2 degrees I quoted was a guessed up number for the purpose of example based on me not recalling the elongations on the 4bd1 pump being large enough for too much more than that.