Quote:
Originally Posted by Stubby79
From what else I've clued in to, it's not just the more modern direct-injection and or common-rail that time the injection at just the right moment. The older, mechanical indirect injection engines do too. They inject it at or a hair before or (as I believe it actually is) after the piston hits TDC, to control when ignition occurs. If it happens too soon, the piston coming up is going to hit all that combustion pressure and bad things are going to happen to the inside of your engine. If it happens too late, well, it might not ignite, or you've just wasted the most efficient time for combustion and lost power.
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It's a lot more complicated than that. You can only get so much fuel sprayed through tiny injector orifices. So the injection period takes a certain amount of time. The faster the engine goes the more that injection timing must be advanced. But just as in a spark ignited engine, changing the injection(ignition) timing not only affects efficiency and even engine longevity, it affects emissions.
Modern common rail diesels not only can precisely control the time the injection begins but can also pulsate and inject a smaller amount of fuel over a longer period of time. This allows for more turbulent air to flow past the injector and receive less fuel, which causes less local enrichment and therefore less NOx and PM formation.
One solution on a mechanically injected diesel that's
running at a constant RPM and load is to size the injector nozzles so that they spray a precise amount over a precise time period at that particular RPM and Load. This might be fine for a pusher engine that runs at such a constant speed and load since the EV can easily add or subtract from that load to maintain a fixed highway speed.