DIYEFI.org Forum

Informative talk on X-Tau transient enrichment by a bonafide expert.

http://video.google.com/videoplay?docid ... 7555&hl=en

Plenty of great information. There's a good part in there about fuel puddling tests at MIT using a 'glass motor'. The clear hardware allowed them to see exaggerated amounts of unevaporated fuel collected in the intake port and combustion chamber on a cold 30°C motor.

- Jim

Edit: Fixed the link.

http://video.google.com/videoplay?docid ... 7555&hl=en

You screwed up the link.

According to the comment that I posted on it in November 07 when I watched it, the first third is dull. IIRC the rest is border-line simplistic too.

Fred.

Ha, I sure did. I fixed it.

Yea it does talk about the various ways we've put fuel into motors over the years. Granted it's a bit simple except for a few slides with the X-tau equations. If anyone is interested they are covered in great detail in US Patent 4,388,906 http://www.google.com/patents?vid=USPAT4388906

Regardless the delivery system, the physics of fuel vaporization doesn't change. I mean, we changed to carburetors, but the issue of liquid fuel in the port and cylinder remains.

- Jim

It just makes me yearn for electric vehicles. Internal combustion is fun but terribly crude in comparison.

I really didn't find that video useful. We all know that things once sucked, and got better with fuel injection. I seem to recall that's called WIG history. This is how we got to grand and glorious now. Little interest in the future, and little interest in the anomalies of the past.

I remember a video showing the difference between direct port injection and manifold injection. It was a video made by Renault and it showed how the performance was increased by squirting directly at the valves. When directed directly at the valves you have next to no XTau effects. When manifold injected, even with XTau compensation, you still have a phase shift and fuel pre-heating. It showed an increase in horsepower and a decreased lag time. This lag is typically tolerable for those of us driving to work. However when pushing for that edge, more power from the same fuel is important. Also that lag is important on a PID system with a control loop larger than just the engine.

Even though XTau helps correct the fuel ratio, it doesn't completely remove some issues. That's one reason why we are seeing a rising trend with direct injection. If your going to do the history thing, attempting to show how XTau is cool, you really should show how XTau has it's place, and is really part of the past.

I think a video of just XTau would have been much more beneficial.

jharvey wrote:I think a video of just XTau would have been much more beneficial.

That's one reason why we are seeing a rising trend with direct injection.

When directed directly at the valves you have next to no XTau effects.

It seemed to me the video was all X-Tau. Just different examples of it depending on the method of fuel delivery.

Direct injection is at a pressure nearly high enough to vaporize the fuel through phase change across the nozzle. That's the only way it can work. Otherwise, there would be unburned fuel in the combustion chamber.

We can talk about that "next to no X-Tau effects", but the motor is going to have the last word.

- Jim

MotoFab wrote:Direct injection is at a pressure nearly high enough to vaporize the fuel through phase change across the nozzle. That's the only way it can work. Otherwise, there would be unburned fuel in the combustion chamber.

Don't forget water droplets are typically preferred for water injection. It's also common to add a soap like substance to change the properties of how the droplets change from drops to gas. That flash from liquid to vapor not only expands rapidly, increasing the pressure, it also takes a lot of energy, which helps keep the temperature down. A combustible flame front is very similar.

Droplets will help slow down the flame front, increasing the amount of time your above the flash point, increasing the chances you have an oxygen around to react with. Bettering your emissions, while giving you more power.

Generally you're looking for enough atomized gas to allow combustion, with the rest of the charge as droplets, that allow for a powerful burn. If you run 100% atomized gas, your flame front will propagate very rapidly increasing the chances of knock, forcing you to decrease your compression ratio. This decreases how much energy you can tap out of the burn. A slower flame front allows you to capture more energy. Kind of like how you typically see an increase in fuel economy with higher octane rated gas. It takes longer to burn, decreasing your chances of knock, as well you get more energy out of each cycle.

Manufactures often use different injectors and injector position to vary the ratio of droplets vs atomized gas. It's kind of an art figuring out how to load the charge you're looking for. Are you looking for pep, or smooth running.

As for a clean burn, that's a different story, where direct injection is king. GDI allows you to insulate the cylinder walls with a wall of unburnt air, decreasing the losses in your heat pump. Ultra lean mode allows for an option you simply can't really touch with fuel injection.

Some draw backs to GDI include a lack of trained mechanics, a lack of field testing / refinement, and different techniques for diagnosing problems. With fuel injection, you can diagnose most problems with a scope, however with GDI, it requires a completely different set of tools. Note I just compaired a tool, to a set of tools.

MotoFab wrote:We can talk about that "next to no X-Tau effects", but the motor is going to have the last word.

When shot directly into the cyl via open valve, there isn't much on the manifold walls to compensate for. Each charge is precisely tuned for each cycle.

BenFenner wrote:It just makes me yearn for electric vehicles. Internal combustion is fun but terribly crude in comparison.

I predict a big market for various MP3 'real motor sounds' plumbed into the sound system. Like ringtones only linked to the gas pedal, I mean encoder. The more expensive models will have 'sampled' motors with multi-note polyphonic.