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The drawback is that higher compression ratios require higher octane fuels. Nissan claims to have solved this by some kind of intelligent feedback system that a
by Aelinsaar 10y ago
The drawback is that higher compression ratios require higher octane fuels. Nissan claims to have solved this by some kind of intelligent feedback system that allows them to control compression ratios on the fly. Presumably the compromise there is that it will require more moving parts, be expensive, and may come with issues of its own.
Variability in something like a combustion chamber is a cool idea, so is variability in the angle of turbo vanes. The issue is how you do it, affordably, and in a way that lasts the way that consumers expect from their cars.
- paulmd 10y agoThe obvious precedent for variable-pitch blades are variable-pitch propellers on aircraft and helicopters. I don't want to say it's a solved problem for turbos, but there is almost a century's worth of prior art on the topic. That's one of those things that (in hindsight) really makes you wonder what took so long to think up. https://en.wikipedia.org/wiki/Variable-pitch_propeller https://en.wikipedia.org/wiki/Variable-pitch_propeller
- seanp2k2 10y agoBecause it has to be cheap and reliable at 1000*C for 200,000 miles over 10 years while constantly accelerating to 280,000RPM and decelerating with no oil or cooling air circulating when the car is stopped. https://garrett.honeywell.com/products/how-a-turbo-works/ https://garrett.honeywell.com/products/how-a-turbo-works/ http://www.epi-eng.com/piston_engine_technology/turbocharger_technology.htm http://www.epi-eng.com/piston_engine_technology/turbocharger...
- Aelinsaar 10y agoThat's a challenge, but the thing is a turbo deals with blazing hot exhaust gasses just out of the manifold. A prop is still a marvel of engineering, but it doesn't need to be made of boron carbide to not melt. A better comparison would be variable pitch vanes on jet turbines.
- _archon_ 10y agoVariable geometry turbos are a thing, and have been for some time. The larger trucks and semis use this technology. Typically they are hydraulically controlled.
- shawkinaw 10y agoRight, I figured this must actually be an anti-knocking technology. My point was just that the quote implied power and economy are optimized by taking the compression ratio in different directions, which is wrong.
- Aelinsaar 10y agoI see, that's a misunderstanding on me then.
- sliverstorm 10y agoThat used to be true, but with modern engine design technique you can get higher than ever compression ratios on standard gas...
- Aelinsaar 10y agoThere are presumably still limits that this addresses, right? Please correct me if I'm wrong, I'd like to learn.
- sliverstorm 10y agoI suspect it does two things- 1) lets the engine have high compression AND a turbo. Typically a turbo is run with lower compression, because even today nobody can make an engine run 14:1 with a turbo. Speculating, when you floor it the compression might drop to 8:1 and the turbo kicks in; then when you highway cruise, it runs 14:1 for efficiency. 2) lets the little engine still hit big torque numbers at low speeds. This is most likely if the engine is variable stroke, as longer strokes improve low speed torque.
- Aelinsaar 10y agoAhhhh, those both make a lot of sense. This could be a nice feature on higher-end cars that try to use smaller engines, with more turbos, for the sake of fuel and emissions then?
- sliverstorm 10y agoRight, I think it's most interesting in the context of the little-engine-big-turbo archetype.
- 13of40 10y agoMaybe the trick would be to have two crank shafts, with two pistons per "cylinder" (or combustion space), and run them slightly out of phase with each other to vary the compression....