Asinine Engineering: Direct Exhaust Injection

Hang around internet fora dedicated to increasing internal combustion performance long enough and you'll eventually run across this type of perpetual-motion misunderstanding. Seems a Vette 'boarder over at the GMModernMuscle Forum needs to study up on several principles before heading out half-cooked, wrench in hand. The brilliant idea that dawned above the poster's head like a 3-watt bulb was that instead of messing around with those ever so inefficient turbochargers, it'd be a simpler route to just run the exhaust back into the intake manifold to put the intake tract under positive pressure. Reading through the multitudinous pages of fodder, this is either someone with "issues" or just a jokester troll.

If he is serious, let's lay out a few simple things here. Firstly, there's not very much oxygen in the exhaust gas, which would make recombustion rather difficult. Second, it's tremendously hot. Third, his assumption that a turbocharger is driven by high-pressure exhaust gas pulses is incorrect. What drives a turbocharger is the expansion of the exhaust gas as it cools and depressurizes. It's the same way an air conditioner works, and it's really thermodynamic theory when you get right down to it. Sure, the exhaust gas is under pressure, but if you think that a turbine is an inefficient way to compress air, pistons are only more so. There's a lot of mass to swing around, and the poppet valves in an automotive cylinder don't improve airflow versus what a compressor housing of a turbo can do, either. Before you raise the EGR flag, let's note that exhaust gas recirculation is used to control combustion pressure and temperature, and since it takes up space that would otherwise be occupied by oxygen-rich intake charge, it's a performance detriment. Finally, how is the engine going to breathe when you've basically set up an infinite feedback loop? We're sure that these sticky engineering challenges are all easily worked out on a bar napkin, though. Why have automakers been denying us the pleasure of this simple high performance option for so long? They must be in cahoots with turbo manufacturers, that's all there is to it.

Thanks for the tip, Richard


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