Diesel vs gas......................

I am curious, though. Why can't you inject the fuel directly into the cumbusion chamber in a gasser the way you do in a diesel? Is there a problem with the way it would burn? I seem to remember reading that detonation is basicly a really messed up flame front instead of a smooth one. Would direct injection of a gasser cause that kind of a problem?

Yes you can. It's been a while since I really paid much attention to the gas side of the world, but I think that there are at least a couple GDI (Gasoline Direct Injection) systems on the road today. I'm thinking a couple Japanese makes and a couple super-luxury European cars. In fact I was reading about some ideas of using GDI to develop a gasoline heavy truck engine for an anticipated economic shift away from diesel in the next decade and a half.

Actually, the concept was fairly sucessful in the pre-carbeuretion days (1930?). There are a couple fundamental issues with it even before you get to reliability questions.

First, a diesel has to use compression ignition due to the higher flash point of the fuel, so it's not as simple as applying diesel principles to gasoline, although there are a number of parallels.

The advantage of current homogenous port injection is primarily one of speed. When you burn the mix, you try to get the most pressure rise out of the combustion as possible. So you initiate combustion just before TDC so the fire is really burning and creating pressure to push down on the piston as it rolls past TDC. The problem is that as the piston moves down, volume is increasing lowering pressure and temperature. So it becomes a race to create as much pressure as possible before the moving piston neutralizes it. Fuel only burns as a vapor, so if it's mixed and a vapor before entering the cylinder, it burns faster than having to diffuse, vaporize and then burn like in direct injection. Faster burns mean more power, which is the inherent strong suit of gasoline spark-ignition port-injection sytems.

(NOTE: Before all the diesel fans get all grumpy, I'm not saying gassers make more power. It's an inherent advantage of the combustion cycle which can be overcome with things like turbochargers, just like fuel economy is an inherent advantage of the diesel cycle.)

Port injection also runs at stoich which makes emissions easier to manage with TWCs.

Direct injection gives finer control over the cylinder mixture so you can create rich zones around the spark plug that will let you run leaner than port injection. Something like 40:1 as opposed to 17:1 for a port system which is where you'd start to lose your ability to ignite reliably. More precise mix control also lets you run at lower rpms (ie.- 600 idle vs 750) and the cooling effect of the direct injection of fuel in the cylinder lets you run higher compression ratios without knock. All this adds up to diesel-like fuel efficiency on regular gas.

However, the same emissions problems of diesels crop up. NOx is high and there can be HC problems is the mixing isn't right. So now you're into higher injection pressures, more EGR, and the ever-challenging NOx reduction in O2 rich environments.

Like I said, I believe it's out there. I think one system is even set up to run both ways. The intake ports come into the cylinder in such a way that they can inject in the compression cycle for GDI operation and fuel economy, and then inject in the intake cycle for homogenous mix and power. Something says Isuzu or Mitsubishi. They've also got a Lean NOx Catalyst (LNC). EGR is used to control knock at the higher homogenous compression ratio. I think at some point, you also have a problem with too much EGR which then requires a high energy plasma or laser ignition system. But I don't think that's part of the systems on the road.

By the way, did you ever get the note I sent you?