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Old 03-04-2014, 02:36 PM   #90 (permalink)
RustyLugNut
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Most of us would like to have an engine development lab in our garage.

Quote:
Originally Posted by RustyLugNut View Post
. . . was one that defined the efforts of the three aforementioned Xprize teams. Each of them responded with different solutions.

The Edison team designed a custom engine that could produce power with a wide valley of low BSFC and also leveraged the generous Xprize rules for specific energy content for fuel volume. Building around the high octane inherent in the E85 fuel, their engine propelled their sleek and light weight car to the class win.

Jack McCornack looked at the requirements for hitting 100 MPGe (MPGe was the Xprize way of equalizing all the fuels and their differing make ups ) and decided on the expedient solution of using a current engine that already had low BSFC over a wide range because of no throttling losses - he chose a turbo diesel engine to power his entry car, "Max". Even with the higher diesel energy factor requiring 114 MPGe, his "napkin calculations", coupled with a sleeker fiberglass body and idealized gearing would have allowed Jack to be competitive if he had not been forced to withdraw due to "production" rule changes.

Aptera's offices and labs were only about half an hours drive from my home in San Diego. I considered them one of the front runners in the 2 seater class before technical difficulties not directly due to their brilliant design left them out of contention. They took the simple expedience of not dealing with engine design or selection and simply went with the most efficient drive train available to them - they used an electric/battery drive.

The disappointing part of the Xprize and the after results is the lack of immediate technology transfer to the everyday world. I had hoped an ICE evolution that would carry forward the momentum of engine tech such as the stratified charge engines from Mitsubishi in the 70s, the lean burn engines of the 80s, and the effective Honda Lean Burn VX engines that came after would make an appearance in the competition. Engine technology would transfer the most readily because consumer acceptance of what is under the hood is very broad - if it works, most people really don't care what powers their car. Unfortunately, engine and drive train development is difficult in comparison to light weight and aerodynamic body design and construction. The backyard tinkerer or the small development group would be hard pressed to compete with the large corporations in the time frame provided. And, no large corporate groups committed to compete.

So where does that leave the average Ecomodder and his equation for BMEP?
But unless you are pgfpro, the shear audacity of the undertaking leaves most people to attack the low hanging fruits of Ecomodding - aerodynamics, weight and rolling resistance. By definition, all Ecomodder's are crazy, but pgfpro and others are reaching for the mountain peak, the last few percent.

There are numerous books and papers that discuss combustion dynamics, but the basics follow the outline to increase flame speed and the heat/pressure release to drive the piston. Flame speed is increased by:

added fuel. Increasing mixture richness or lower AFR.

added pressure. Flame velocity is lower at partial throttle, higher at full and boosted levels - all else being equal.

added turbulence. Flame velocity is increased with added port tumble, squish and piston dome shaping.

added heat. Especially at the spark ignition point, a hotter fuel mix contributes to flame speed.

added chemical reactivity. Racers have used various compounds over the years to not just fuel, but change the engines power profile. Nitro-methane and hydrogen peroxide are just two that have been used.

I will briefly discuss the first four points as a group and then the last point in a more extensive fashion as that relates to the original post. As both Niky and Tvago have pointed out, water injection and EGR change the chemical reactivity rates and surprisingly to some, they can be used to accelerate and stabilize certain flame situations when all logic tells us they would quench the flame.

Last edited by RustyLugNut; 03-04-2014 at 02:39 PM.. Reason: punctuation.
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