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Variable compression? So logical next step is variable octane, right? One tank of 87 and one tank of 93, dynamically mixed in response to changing compression ratio. :)
A common way to run lots of boost on turbocharged cars is to use 87-93 normally, and E85 under high boost. You get the high octane when you need it, without the 30% hit to fuel economy when you don't.
Anecdote: I'm running a water/alcohol injection system on my 335i (tuned on higher than stock boost). It really helps on the hotter days. Currently, I'm running a mixture of ethanol (everclear) and distilled water. Most people use methanol, but it's not really compatible with the viton gaskets inside of my pump.
SAAB was working on one of these years ago before GM flew that company into the ground. This link gives you an idea of how it works. https://en.wikipedia.org/wiki/Saab_Variable_Compression_engi...
Earlier quoted context omitted.
not sure if joking or not, but the injection map on your ecu does that.
I think you are incorrect. Most production cars (all that I know) have only a single fuel tank, thus nothing to mix.
How are they preventing engine knock ? At 14:1 compression you'd have to be running premium gas, which would pretty much cancel the efficiency saving, in a $$ perspective.
How are they preventing engine knock ? At 14:1 compression you'd have to be running premium gas, which would pretty much cancel the efficiency saving, in a $$ perspective.
I'm curious as to what, if anything, would preclude variable displacement from being used in diesels as well as petrol engines. Diesels also have performance/economy tradeoffs like this gasoline engines, so it stands to reason that tweaking compression should give similar benefit. I've got a feeling that rumors of diesel's impending demise are greatly exaggerated.
Diesel's fatal flaw is particulate emissions. Attempts to mitigate those, without cheating (e.g., VW), have proven difficult.
From the (much better) Reuters article [0]: > The higher the ratio, the more efficiently the engine works, producing better fuel economy and, with the addition of a turbo-charger, more power. > Traditionally, design engineers had to fix a gasoline engine's combustion compression ratio, essentially deciding whether to go for power or economy. Everything gets better with higher compression ratio, so where's the comprom…
Higher compression ratio = better combustion = more power with less fuel, regardless of turbo.
Higher compression will always mean better fuel efficiency. My moms Mazda CX-5 runs a 13.5 compression ratio and gets 27mpg. The power/efficiency trade off appears when you're talking about turbocharged engines.
Turbos compress air into the combustion chamber, so when you compress compressed air, you get exponentially compressed air. If you don't adjust the compression ratio for the turbo, you're going to get knock. An engine with a 13.5 compression ratio can run perfectly fine on regular gas, but an engine that runs the same ratio and 20 psi of boost with melt itself even on premium gas.
This is a problem for any kind of work vehicle. They almost always have turbos to give them the power to haul their loads with reasonable ease. But this forces the engineers to design the engine with a lower compression ratio, which means at light engine loads, when the turbo isn't doing anything, the engine isn't fulfilling its full potential. It's still compressing air at a low ratio, even though it's not taking in already compressed air. This is where Nissans innovation comes in. The compression ratio will change according to the degree with which the turbo is compressing air. Allowing the engine to run at peak efficiency through the entire range of engine load.
Here is a simple explanation gathered from this image http://o.aolcdn.com/hss/storage/midas/de3e84cf46030cd7b3ea39... : The difference from a conventional engine is that the engine stroke is increased or decreased automatically. The stroke is the distance the piston travels up and down in order to turn the crankshaft. A shorter distance that is closer to the combustion chamber increases the compression ratio because…
Haha, this reminded me of a friend who did a summer engineering internship with a U.S. automaker. He came back and related one of the most important things he learned: "never, ever buy a first-run car." I remember hearing him tell about tolerances still being adjusted after all these cars had shipped, cringing to think of all the problems that could cause.
I have a lot of respect for Nissan's engineers, but somewhat less respect for their lawyers, who hounded Uzi Nissan for a decade trying to get him to turn over his domain. http://www.nissan.com/Lawsuit/The_Story.php Cybersquatting is obnoxious, but once they found out it was the guy's actual name, and that he was using it to run his own business, they should have just accepted he had a right to the site and worked to…