Turbo Questions

[schoelta]

I just thought I would pick your brains for some ideas. I am
looking at installing a turbo on my truck. Doing some
research, I found that it's really hard to run more that 8-9
psi of boost on these engines because of the high
compression ratio. I understand that the newer trucks are
running 17 or 18:1 compresstion to allow for more boost.
There are low compression pistons I can buy for my engine,
but I have been told that this will cause hard starting for
these engines. My question is why? I understand that a
diesel need compression to fire, but how are the new diesel
different that allow for easy starting with lower
compression. I'm sure the computer helps, but what exactly
does the computer to do the engine to make it start so easy?
How did Cummins achieve a good starting engine with 17.5:1
compression and NO computer back in '89? Did they have the
glow plugs stay on longer?

I'm not looking for a parts list of things I will need for my 6.9. I am looking for the theory behind the hard starting issues. What do the other trucks specifically do, or what differences are there that make these engine start with lower compression?

Tim

 

[sassyrel]

the powerstrokes are DIRECT injected--and thats why they start better--even with lower compression--the injector tip is in the "cup" of the piston--right where the heat is------

 

[flatlander]

Just guessing, but the higher the compression the hotter the air gets during the compression stroke, so the fuel is injected at a lower psi. Ours is somewhere in the 1800 psi range. I think that powerstrokes have 10,000 psi fuel pressures (give or take) so it doesn't need as high compression of the incoming air.

I'm sure that someone has a better grasp of the numbers and the science but that's my take on the subject.

 

[sootman73]

right, air(gas) heats up as it is compressed. especially to the pressures seen in the combustion chamber. i know the psd inject fuel at around 22,000 psi! or in that pressure range.

anyway the key is direct injection! thats how they manage with the lower pressures and higher boost. plus they are much easier to cool as the heat is not located in the heads but in the cylinder itself. thats why it can be hard to keep the idi's cool. whether it be a 6.2, 6.5, 6.9, 7.3.

 

[h2odrx]

so say we lower the compression, add more boost, and say add propain or nitrous? and then delay the glows to come on after it starts cranking? would that be a big BOOM????

 

[RLDSL]

These newfangled things not only are direct injected at higher pressures, but their computers vary the injection timing for the cold starting and they also generally inject more than one pulse of fuel per stroke depending on the engine. There's a lot of factors involved that allow them to run at lower compression and run higher boost.