Displacement on Demand
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From: Houston, Texas
Displacement on Demand
Could Mazda impliment a DOD function on the rotary for gas savings while cruising?
I figure if they step up to a three rotor for the RX7, gas mileage would really start hurting, but this could be a great way to maximize performance and gas mileage, I just don't know if you can shut down one rotor.
I figure if they step up to a three rotor for the RX7, gas mileage would really start hurting, but this could be a great way to maximize performance and gas mileage, I just don't know if you can shut down one rotor.
mostly harmless
Joined: Jun 2002
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From: Greater Vancouver Area, BC
this was discussed a while back, and there's almost no chance of them using a 4 rotor motor for the next RX-7 (assuming one's even coming), so dont' worry about it.
DOD is a complicated technology, and is applicable to GM's push-rod engines only as they somehow disconnect the lifter from the pushrod to keep the valves from opening.
to do the same thing in a wankel engine, you'd have to somehow block the ports on alternating cycles... ****, i worked this out before, and it gets really ugly with the firing order...
as it happens, the usual order is:
F R F R F R and if you were to start skipping them out, you'd get all kindsa wonky crap happening. could they run it on only one rotor?? that might be an idea right there, but it wouldn't work well at idle/lower rpm, and it certainly wouldn't be any good at higher rpm.
DOD is a complicated technology, and is applicable to GM's push-rod engines only as they somehow disconnect the lifter from the pushrod to keep the valves from opening.
to do the same thing in a wankel engine, you'd have to somehow block the ports on alternating cycles... ****, i worked this out before, and it gets really ugly with the firing order...
as it happens, the usual order is:
F R F R F R and if you were to start skipping them out, you'd get all kindsa wonky crap happening. could they run it on only one rotor?? that might be an idea right there, but it wouldn't work well at idle/lower rpm, and it certainly wouldn't be any good at higher rpm.
Displacement on Demand is a huge gimmick
If your cruising down the road at say a constant 50 mph, you have to make the same amount of hp no matter how many cylinders you are running.
For example, say your car needs to make 20 hp to cruise at 50 mph. A conventional V8 would need very little fuel per cylinder. Now DOD kicks in, and most people assume now you are getting the economy of a 4 banger. WRONG! While it does turn the V8 into a I4, it still needs to make 20 hp to keep the speed. So the rate of fuel per cylinder must increase since you just lost 4 cylinders. This doesn't sound that bad at first, but now you are lugging around 4 dead cylinders and all of their rotational inertia and frictional losses plus you have to fight a bit of compression (eventhough the valves are open).
So in reality, your turn a pretty efficient V8 into a very poor I4. Thats why GM can't get more then a 5% increase of efficiency out of DOD. To me, 1 extra mpg isn't worth the increase of price and stuff to go wrong.
If your cruising down the road at say a constant 50 mph, you have to make the same amount of hp no matter how many cylinders you are running.
For example, say your car needs to make 20 hp to cruise at 50 mph. A conventional V8 would need very little fuel per cylinder. Now DOD kicks in, and most people assume now you are getting the economy of a 4 banger. WRONG! While it does turn the V8 into a I4, it still needs to make 20 hp to keep the speed. So the rate of fuel per cylinder must increase since you just lost 4 cylinders. This doesn't sound that bad at first, but now you are lugging around 4 dead cylinders and all of their rotational inertia and frictional losses plus you have to fight a bit of compression (eventhough the valves are open).
So in reality, your turn a pretty efficient V8 into a very poor I4. Thats why GM can't get more then a 5% increase of efficiency out of DOD. To me, 1 extra mpg isn't worth the increase of price and stuff to go wrong.
Hyper Space RX-8 _,.-^'`
Joined: Aug 2003
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From: Motor City
Now, if they could set it up as a secondary clutch/flywheel combo to engage/disengage it, that would be something,
but that's basically two complete engines inline.... a dual V4's anyone?
OverLOAD
but that's basically two complete engines inline.... a dual V4's anyone?
OverLOAD
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From: portland oregon
ha i thought keech,buger and i wouldn't ever see this topic again. we had it worked out awhile ago how it could work. when dan from rotary news had a chance to talk to some folks from mazda he put the question to them and they basically said they would never do it. it's not a direction they are even thinking of looking. they are looking more towards the hydrogen duel fuel or hydrogen hybrid direction. but search mine and wakeech's posts for the old thread
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