: Direct Injection - Technology Question

10-26-11, 01:21 AM
I remember reading about direct injection many years ago and
didn't think much about the advantages, but I notice now that
engines have come out with very high HP/liter that employ
direct injection. I noticed many run on regular fuel yet
with compression ratios around 11.4 to 1. The GM LLT engine
used in the newer STS is one example:

Does anyone know if they time the injection just before the
time of ignition and past the time that would be pre-detonation
to avoid this condition?

I doubt there is anyway to convert the older Northstars to
direct injection since there is no place for the injector in the
head. Are custom heads the only way? Not exactly practical.

Will's Fiero
10-26-11, 08:46 AM
Direct injection atomises the fuel better than port injection. The attendant evaporation cools the intake charge and allows higher compression ratios.

The fuel can be injected during the intake stroke and into the early portions of the compression stroke.

When you put aggressive cams with a long intake/exhaust overlap period in a port injected engine, some of the mixture will get scavenged during the overlap period and raw fuel will come out the exhaust. This is an emissions no-no, of course and dictates the maximum state of tune that a port injected engine can have while staying clean enough to pass OEM level emissions.

In a direct injected engine, the injector doesn't start spraying fuel until after the exhaust valve is closed (control strategies for this with variable cam phasing get interesting), so there is ZERO possibility of fuel going out the exhaust. This means that direct injection engines can have very high states of tune (high HP/litre) and still have good emissions and driveability.

The LLT is about 85 HP/litre. At that state of tune, a Northstar would have 390 HP and the 5.0 DI that GM *was* working on would have 425 HP.

I wouldn't want to bother converting an older Northstar to DI, but the idea of doing it to a VVT LH2 Northstar had crossed my mind. If all the hardware can fit in the space available, then it would obviously require machine work on the heads... difficult, but not impossible. Next step would be packaging the injectors in the valley, which would require custom fuel rails to handle 2000 psi fuel pressures and a custom intake manifold to clear all the parts. After that, figure out how to drive the high pressure fuel pump.

By the time I actually get around to doing this, aftermarket engine management for DI will be available (MoTeC is working on it now) but will probably end up being the majority of the cost of the conversion.

10-26-11, 10:53 AM
Thanks, very interesting! I'm surprised that they went to supercharging in the LC3,
when they had the technology and parts for direct injection in the LLT given the
complexity and reduced reliability with a supercharger.

Will's Fiero
10-26-11, 04:06 PM
The supercharger would give them far more torque than the DI, and was a more mature technology.
DI engines have had many teething problems, from carbon deposits on intake valves to fuel pump failures on BMW's...

11-04-11, 01:37 PM
Direct injection has been around for quit awhile now, just not in cars. My 1999 Mercury Optimax outboard runs great and hasn't had any issues after the recalls. Very reliable

Will's Fiero
11-04-11, 03:17 PM
Does your Mercury have PCV *AND* EGR?

11-07-11, 05:33 PM
I had actually heard about it in the 1970s but mainly for use in diesel engines,
I'm not sure but I thought there were also some European cars that used it for
gas. There is some history here and it seems that it became more popular in the
mid to late 1990s: