Ethanol injection for knock control

[theholycow]

http://www.newscientist.com/article/...efriendly.html

[...]So says the Ford Motor Company, which on 19 May revealed test results on a novel ethanol-assisted engine. Called a direct-injection ethanol engine, the unit runs primarily on petrol. When it needs to deliver maximum power - to climb a hill or overtake, for example - the engine management computer adds a little ethanol to the fuel injected into the combustion chambers.

This arrangement allows the engine to operate at a much higher compression ratio - a measure of the amount by which the fuel-air mixture is compressed before being ignited - than normal. As a result, an average car engine can be "downsized" to one that should have around 23 per cent better fuel efficiency, Ford says.[...]

Nothing particularly new to us here, but a different combination of things we often talk about.

[GasSavers_maximilian]

They mention that they want to make sure the engine doesn't become unusable if you run out of ethanol. Seems like in that case, you could limit maximum power to prevent knocking. Not exactly ideal, but you could still limp along.

[GasSavers_Erik]

Is the compression ratio adjustable on this engine? That seems like the only logical explanation for why it doesn't detonate without the ethanol- perhaps it is a turbo and when the raise the boost pressure way up, they start injecting more ethanol.

[GasSavers_maximilian]

If it were variable compression they wouldn't need ethanol for knock control. They're worried that it will knock without ethanol. Knock usually is much more of a problem at higher throttle settings.

[GasSavers_Erik]

I see- so it is simply a high compression engine that adds ethanol only under high throttle conditions (that normally would result in knocking).

This would make fillups interesting- have two gas gauges, tanks, filler caps, etc.

[greasemonkee]

WOW! How about stepping back to the thirties?

IF an auto manufacturer were attempting to create an economy engine via high compression, seems like the most logical thing to do would be to properly design a combustion chamber that would withstand 12-13:1 static compression on pump fuel alone, it is a fact that as static CR #'s increase, maximum cylinder pressures greatly increase. Over the long run this is probably not desirable.

For starters, ethanol has a low calorific value - the stoichiometry is 9:1, so obviously not the most potent fuel for extracting mechanical energy. The only thing beneficial to ethanol as a fuel supplement is its high latent heat absorbs massive amounts of heat before the intake valve closing resulting in a higher volumetric efficiency, (more power). However, this may only be true to an extent, under certain conditions and there is a point of diminishing returns. The slight gain in thermal efficiency is likely at best, negligible for this company's approach.

Reducing flame temps increases thermal efficiency. This has been commonly accomplished by operating at leaner than stoichiometric ratios with a super homogeneous intake charge. There is a catch though, flame speed decreases and more ignition timing is required, ultimately defeating the purpose. The static CR could be increased to offset the difference, I find it a curiosity as to why this hasn't been done. The civic VX is likely operating in that sweet spot somewhere between 16-17:1. Nearly every mass produced powerplant after that engine which attempted to achieve ultra high economy is severely lacking.

[theholycow]

Originally Posted by greasemonkee

For starters, ethanol has a low calorific value - the stoichiometry is 11:1, so obviously not the most potent fuel for extracting mechanical energy. The only thing beneficial to methanol as a fuel supplement is its high latent heat absorbs massive amounts of heat before the intake valve closing resulting in a higher volumetric efficiency, (more power).

Were you talking about ethanol, or methanol?

[greasemonkee]

Ethanol. Must have been thinking one thing and typing another. Stoichiometry of methanol is 6.4, ethanol is 9.0

Went back and edited my #'s, haste = errors. My apologies

[imzjustplayin]

Originally Posted by theholycow

Were you talking about ethanol, or methanol?

lawl, you're funny, you found the methanol problem in my thread and then in this thread... I wonder how many times this happens on this forum.