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Here are a few questions I've been wondering about. I've tried googling and Wikipedia, but haven't found the answers. I figured you guys probably know this stuff...


1. In fuel injected cars the fuel feed is shutoff during engine braking. I know this is true with cars that have manual transmission, but is it true with cars that have automatic transmission as well?

2. I know that cars with automatic transmission should not be flat towed (e.g. all wheels on the ground), because it damages the automatic gearbox. Why is that exactly? What is different in that situation compared to normal driving, assuming the front of the car is to the forward direction while towing? The engine and the flywheel still rotate, right. Why not just put the shifter to "D" and then tow the stricken car?

3. I'm still not 100% sure what the "L" mode is used for. In my 1998 Chrysler Voyager there is P R N D 3 L. The manual (in Finnish) says that the "L" is to achive maximum engine braking and that upshifting does not occur until the max RPM has been reached. And also that downshifting happens earlier. Hence the maximum engine braking. In wikipedia the L mode is described as just locking the first gear. In my Voyager does "L" use all the four gears? Does the "L" stand for low gear or low gears?

4. I know that in general, automatic transmission is less fuel efficient than manual transmission. Is this mainly due to the torque converter slipping? Or is it because manuals usually have five gears compared to three or four in automatics?


Thank you in advance

Jake

 

[ol][*]Not sure about that, but I doubt it. You want to maintain a little engine torque to keep everything pumping fluid in the right direction. See number 2.[*]In the old days, (1950's), automatic transmissions used to have what was called a "rear pump" that could maintain fluid pressure when coasting. It also made it possible to actually push start a car with an automatic transmission. That was removed, mostly to save costs, with the result that unless there is input from the engine, you don't have proper fluid pressure and circulation in the transmission. The engine and flywheel do NOT rotate when towing, even if in gear. So putting it in gear and towing it won't provide lubrication in the transmission unless the engine is running. Also, it's not good to put the car in neutral and coast down long mountains. Now you see why they are unlikely to shut off the fuel during engine braking conditions with an automatic transmission. With a manual, you want to minimize backfiring, so fuel shutoff is called for.[*]So why is there an 'L' range? Good question. Before computers, 'L' would lock it it first gear. With today's computer controlled engines and transmissions, the ultimate decision of which gear is left up to the computers, not the driver. It seems that the selection of 'L' with the shift lever is a 'suggestion' that you, the driver, would prefer a lower gear if that's all right with the computers.[*]The answer to your question is 'yes'. (Both are responsible for a contribution to the loss in fuel economy.)[/ol]

 

Your Voyager will use all 4 gears only when in "D"rive. When in 3, it will use 1,2,and 3, eliminating the overdrive 4th gear. This is useful for lower speed in town type driving, when towing or heavily loaded, or in hilly country where it tends to shift too often or the motor lugs down before shifting out of overdrive. It will also provide a modicum of engine braking above 40 mph. Below 40 the rpms just aren't there to support it.

The "L"ow position is usually very ineffective as it is completely computer controlled exactly as the manual has stated. The problem, it shifts down to first gear too readily and too early, and too harshly for my taste. I have found it useful though for hauling very heavy loads up very steep grades. My Jeep Cherokee Sport 4.0 at 3500 rpm in L (second gear) runs very strongly up steep grades hauling a very heavy trailer at about 50 mph. (torque converter locked) It does this at about half throttle causing very little overall stress on the vehicle. The 3500 rpm range is usually very powerful and easy for most modern engines to perform for long enough to get the job done without undue wear or damage.