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Fuel Consumption - hare-brained theory

 
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Shrapnel



Joined: 20 Oct 2005
Posts: 16

PostPosted: Tue Feb 21, 2006 5:25 am    Post subject: Fuel Consumption - hare-brained theory Reply with quote

I whole-heartedly believe in the "smiles-per-miles" philosophy, but fuel consumption and emissions is nevertheless a significant and interesting subject. Is there any validity to the following theory?

1. The speed of the rotor varies with RPM (duh!).
2. We know that the speed of the rotor outpaces the fuel burn, resulting in fuel economy penalties, and unburned hydrocarbons.
3. But surely the fuel mixture burns at the ~same~ rate, regardless of the rotor speed.
4. Therefore, the lower the RPM, the greater the amount of fuel mixture actually gets burned, because there is more time in the chamber per ignition for the burning to occur.
5. Therefore, at low revs, increased economy is not only the obvious result of injecting less fuel into the chamber in the first instance, but overall efficiency is also increased due to a greater percentage of the available fuel actually getting burned, rather than getting ejected unburned.

Would this be right?
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Tony Shrapnel
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Blake
Been there, done that


Joined: 12 Jul 2005
Posts: 135
Location: Portland, OR

PostPosted: Wed Feb 22, 2006 2:11 pm    Post subject: Re: Fuel Consumption - hare-brained theory Reply with quote

Shrapnel wrote:
I whole-heartedly believe in the "smiles-per-miles" philosophy, but fuel consumption and emissions is nevertheless a significant and interesting subject. Is there any validity to the following theory?

1. The speed of the rotor varies with RPM (duh!).
2. We know that the speed of the rotor outpaces the fuel burn, resulting in fuel economy penalties, and unburned hydrocarbons.
3. But surely the fuel mixture burns at the ~same~ rate, regardless of the rotor speed.
4. Therefore, the lower the RPM, the greater the amount of fuel mixture actually gets burned, because there is more time in the chamber per ignition for the burning to occur.
5. Therefore, at low revs, increased economy is not only the obvious result of injecting less fuel into the chamber in the first instance, but overall efficiency is also increased due to a greater percentage of the available fuel actually getting burned, rather than getting ejected unburned.

Would this be right?


No. 2 is not quite right. The gas flow inside the chamber exceeds the rate of propagation of the flame front. The rotor is quite slow. In fact, the rotor is rotating at 1/3rd of the forward rate of the eccentric shaft, though it orbits at full speed (you can read about this on the main site). Also, the engine has a 270 degree stroke, comparred to the 180 degree stroke of a piston engine. Plenty of time to burn fuel. The problems are the chamber shape, the surface area, gas flow and port location. Mazda has done quite well reducing emissions by implimenting a wasted spark setup on the leading ignition (86+) and moving the exhaust ports to the side plates (RENESIS), as well as refining the EFI, yadda yadda.

Efficiency of internal combustion engines is typically maximized at Wide Open Throttle (WOT). That's why small displacement engines tend to be more fuel efficient...they spend much of their lives at or near WOT.
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