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Old 03-07-2009, 01:29 PM   #7 (permalink)
Engineering first
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Originally Posted by roflwaffle View Post
A rich mixture is used to maximize power output and cool the intake charge in order to minimize the risk of detonation, not cool the catalytic converter, which has typical operating temperatures around 1500-1600F, right around the maximum EGTs SI most engines will make.
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With conventional four-cycle engines, there are times when fuel enrichment becomes necessary to cool the exhaust gases to prevent degradation or destruction of the catalytic converters. With the Atkinson cycle, the expansion/power stroke is longer than the compression stroke so that combustion energy can more effectively used for production of engine power. This results in lower exhaust gas temperatures.

In the process of re-circulating exhaust gas, the cooled EGR system increases the specific heat capacity, also resulting in lower exhaust gas temperature. Regulating the amount of EGR can also control the exhaust gas temperature.

The combination of the Atkinson cycle and cooled EGR minimizes the need for fuel enrichment. The benefit is significant reduction of fuel consumption, especially during high-load driving (e.g.: hill climbs and freeway driving.)
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When I had my 150 hp, Cherokee 140, we used a rich mixture at maximum power settings so it wouldn't burn out the valves. For maximum power at cruise, the instructions were to trim it 50F rich under peak. BTW, there is an excellent write up on "Exhaust Gas Recurculation" from AutoSpeed.

Originally Posted by roflwaffle View Post
Having the larger 1.8L engine and taller gearing could very well help out with SFC, although the four-stroke Atkinson cycle is exactly like the SI Otto cycle minus the delayed closing of the intake valve after the piston has started to travel upwards again. . . .
It means:
  • 8-to-1 compression stroke - in an Otto engine, the throttle plate would cause a lot of pumping losses in low power regions whereas the Atkinson cycle makes a substantial reduction in throttle plate losses. This is especially useful in low power regions where low-drag vehicles cruse.
  • 13-to-1 expansion stroke - provides a high expansion ratio so a large percentage of energy is extracted. Only diesels have a higher expansion ratio but they also have a problem with NOx formation. Longer durations at higher temperatures favors NOx formation.
Modern Atkinson cycle engines have different compression strokes from the power expansion stroke. This means less maximum power but substantially improved brake specific fuel consumption. Some of believe that the variable valve stroke and angle on intake and exhaust valves, already in some Toyota vehicles, will complete the picture and bring their Atkinson cycle engines into diesel BSFC ranges without diesel problems.

Bob Wilson
2019 Std. Range Plus Model 3 - 134 MPG3 || 2014 BMW i3-REx - 117 MPGe, 39 MPG
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