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Views: 2,277  ·  Replies: 6 
> Compression Ratios, A quick saunter into this.
DigiBunny
  Posted: Oct 13 2010, 08:44 PM


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So I was reading an Article on the Sky-G and Homogenous Charge compression Ignition on Wiki, and I must say I'm fascinated and stumped as to why this has not been implemented on a grander scale along with variable camshafts to increase fuel economy. It sounds like that with HCCI, every car out there that reaches the optimal operating temperature will start running like a diesel.

But more to the point. I'm attempting to run a quick hypothetical calculation from a car's
mpg in the city compared to it's fuel ratio.

Now normally I'd just compare the ratios and even things out, but I'm missing two factors that would throw off my accuracy quite a bit; namely:

1.) An accurate guestimate as to how much more a gasoline engine (Like the Sky G) would increase in it's air-fuel mix. If possible, I want to play with rotaries too, but I'll leave it at normal piston engines.

2.) Whether the relationship between mpg and compression ratios scale linearly.
MetalMan777
Posted: Oct 14 2010, 01:40 PM


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Maybe it's just me, but if you want an engine that runs like a diesel, why don't you... buy a diesel?

Political commentary: who was the @*^$head who decided emissions were more important than economy? Ralph Nader, was that you? Evil @^%$headed (*^%face.

Um, I really fail to see the advantages of HCCI over a straight diesel. Not only can you run a diesel engine on diesel, but veggie oil, coal dust, or pretty much any combustible material that can be sprayed in fine particles. A HCCI gas engine is pretty much going to limit itself to gas.

Now to actually be on topic and answer your questions:

1. This forum has some data on a Honda K series engine. It seems to indicate going any leaner than 15.7AFR actually increases fuel consumption.

2. No, the formula for MPG is a complex one involving mechanical friction, atmospheric friction, and thermal engine efficiency at particular rpms. It's complicated, I'd take gearing and friction out of the picture and simply compare engine thermal efficiency and compression ratio. If you take adiabatic index out of the equation, then yes, compression ratio is directly proportional to thermal efficiency. You can't do that, though, so in practice it's a direct relationship, but doubling your CR will not quite double your efficiency.




Also important: Burning lean is burning hot, high performance engines almost always burn rich, not just to decrease adiabatic heat, but because ideal ignition characteristics are wildly different between low and high rpm states. Just look at a Diesel's redline, they're almost never very high. HCCI/Dieseling/preignition/knock is the enemy of the high performance gas engine.

This post has been edited by Cactus on Oct 14 2010, 01:41 PM
DigiBunny
  Posted: Oct 15 2010, 06:43 AM


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True enough. There's certainly more sense in using a diesel if you want a diesel. But then again, I see this technology doing very well in crawling traffic that moves an inch every 5 minutes. In the Philippines, the only diesels I see around here are big, big cars which are not pleasant to use in a city where road congestion is pretty much mandatory.

Couldn't an ECU be configured to run at different ratios, be it manually or some other trigger dependent on the driver's actions?
MetalMan777
Posted: Oct 15 2010, 12:50 PM


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The primary job of the ECU is to modify afm with revs and engine load. At full throttle high in the rev range, you want to run richer than you do at low rpm's and part throttle. Forced induction cars want to run very rich at high rpm, to keep out diesel-like detonation and lower the adiabatic temps. If the ideal afm was the same throughout the rev range, we'd never have moved away from carburetors.

If you want to tune your own ecu for either mileage or power, I recommend getting a standalone programmable unit. You drive a Toyota, so I'm willing to bet some of the major tuners like AEM have plug and play systems for your car. I'm going to purchase a Megasquirt system in the coming weeks, and I'll be installing it in my 6 series. If you go this route, I recommend you get a wideband lambda sensor that supports datalogging, all the good ones do. I'll be getting an innovate LC1 with a guage, so I can keep an eye on AFM as I drive. I'll post about it in my project thread, I'm far overdue.
DigiBunny
  Posted: Oct 15 2010, 08:21 PM


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So if an ECU DOES change the Air/Fuel, then I'm having trouble understanding why an engine can't run lean at one point, then rich past a certain RPM.

For the record, I have next to zero in the technical skills department when it comes to this. I'm only going into theory about this.
MetalMan777
Posted: Oct 16 2010, 09:54 AM


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They can run lean at one point, and then rich given a certain input. Manufacturers tune their cars to run on crap gas, so that if it's supposed to run premium, the knock sensor will compensate if you fill up with regular. Almost nobody tunes engines to run lean. Most tune to around stoich 14.7:1 near an idle and at low rpm, taking it richer as you increase throttle and rpm.

http://en.wikipedia.org/wiki/Lean_burn

Read that, eco freaks go on and on about the honda lean burn engines.
Spaz
Posted: Oct 16 2010, 03:35 PM


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ECUs also adjust to the fuel you put in the car. I'm currently on a cruise, and had to put 91 octane in my car before I could find 93. After a couple moderate pulls, the car stopped knocking as the ECU pulled timing back a bit.
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