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That makes really good sense.
Real world gas mileage?
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So I can't stop thinking about this concept and how it operates in real life. My issue lies in that it sounds so much like the MDS in the Hemi; when the cylinders are not needed, the fuel supply is cut and the Jeep gets more MPG's. But what I can't figure out is how (or why) the MDS is more complicated. Is everyone with a computer-controlled vehicle essentially driving a MDS vehicle, or is this difference some testament that the fuel is not really choked off when coasting in gear?
Rob, you got this one?
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If I understand your question properly, no ... the Jeep MDS system closes the exhaust and intake valves on 4 of the cylinders when it's activated.
The task is achieved by collapsing (reducing in height) all 8 of the hydraulic valve lifters on the 4 cylinder in question, by use of an electric solenoid that reduces oil pressure within the lifters
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So the timing is set up to run smoothly on half the cylinders, and the solenoid determines if the "additional" valves operate? What about the air inside the not-in-use cylinders? Does it just get repeatedly compressed?
Back to the original question, do you concur that the non-MDS engines completely cut the fuel supply when coasting in gear?
Back to the original question, do you concur that the non-MDS engines completely cut the fuel supply when coasting in gear?
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That is outside my area of knowledge .. so i just don;t know
No adjustment to timming is needed (for the MDS to function) the cylinders that are closed just get no spark ... and 8 cylinder engine is naturally balanced already.
Yes the cylinders that are closed operate as "air springs", but bear in mind each cylinder that is compressing, is perfectly balanced by a second cylinder that is expanding, on the opposite side.
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If the fuel were completely cut off the engine would stop running.
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It takes a real man to say these words!
The "air springs" make more sense since you reminded me of their counter-balance, and likewise on the V4/V8 ignition.
Still, I would love to understand how these newer engines can supposedly just shut off the fuel when coasting. It just doesn't seem right.
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...and this was an argument addressed in the article (which I have looked for, but not found). Their reply was that yes, the engine will stop firing, but the kinetic energy would keep everything revolving. Their example and suggested experiment was to shut off your ignition while driving and be amazed at the smooth continuation of your vehicle along the road.
I have not done this.
I experienced a bad tank of gas while doing 100mph on the highway once and it was anything but a smooth continuation. It was a lightweight, manual transmission coupe, but I still believe it would be unpleasant in heavy (kinetic) vehicle with an AT, aka the Jeep.
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Oh wow!
Everyone is exposing themselves to brain damage thinking about this subject.
Regarding steady speed operation on a 5.7.....a oil relief passage, within the block, is controlled by the ECM.
When all the MDS parameters are met the ECM opens the relief passage allowing all pressurized oil within the 8 lifters related to the MDS cylinders to bleed off.
The lifter base still rises and falls while following the camshaft, but because there is no pressure within the lifter, the piston within the lifter just floats, holding the pushrod in position while lifter rises and falls around it.
All 8 valves are closed and the pistons operate in a pressure/vacuum environment.
Beyond the parasitic drag of rings and bearings the pistons neither contribute to nor cost any measurable power.
At this time the injectors are also shut down and the coils are switched off.
So much for MDS...
Carbureted engines produced huge amounts of unburned hydrocarbon emissions (unburned fuel) because when you took your foot off the throttle and coasted, the vacuum below the closed throttle plate pulled fuel through the idle circuit.
Because the throttle was closed there was not enouph air below the throttle plate to support combustion.
As a result, unburned fuel just passed through the combustion chambers and out the tailpipe.
The worst examples were vehicles with manual transmissions because the RPM could not drop due to the fact the car was now turning the engine, through the clutch. But the automatics were almost as bad as they kept the engine spinning as well.
In the late 60's manufacturers introduced a method to inject a predetermined amount of air into the exaust manifolds, helping to finish burning the unburned fuel being pumped out through the exaust manifolds.
Old car guys know this as a Smog Pump.
It was nothing more than a belt driven low pressure high volume air pump using a diverter valve to meter air into the system and turned out to be quite effective at reducing unburned hydrocarbons throughout the RPM range.
Because it looked like it might cost horsepower....we threw them out by the trainload.
Anyways, control of unburned hydrocarbons at coast has been a engineering nightmare from the time we knew they were a problem.
Electronic fuel injection has just about turned this into a non concern.
Even though the vehicle is turning the engine at speed during coast (foot off throttle...say 60 MPH for example) and the throttle plate is closed, the ECM has shut the injectors off and the engine is nothing more than a big vacuum pump.
As long as the front pump within the trans is turning, it will transfer rotation from the wheels back through the driveshafts, trans, torque converter, and ultimatly the crankshaft.
If you just have to experiment, find a vacant section of road (fat chance) run up to about 60, release throttle and turn key to off. Do not take it out of gear.
Your going to have to guess your speed cause the dash will be dead during this experiment, but at about 30 mph turn the key back to ON (not start).
I haven't done it with my Jeep because....I'm not that inquisitive, but, you likely will find the engine is now running.
You have now manually done what the vehicle does electronically....you shut off the injectors during deceleration and restored the injector operation just as the front pump was loosing its ability to drive the engine due to lowering road speed.
So all that aforementioned B/S that preceeds boils down to is...YES the injectors shut off for a period of time during deceleration.
Rob
Everyone is exposing themselves to brain damage thinking about this subject.
Regarding steady speed operation on a 5.7.....a oil relief passage, within the block, is controlled by the ECM.
When all the MDS parameters are met the ECM opens the relief passage allowing all pressurized oil within the 8 lifters related to the MDS cylinders to bleed off.
The lifter base still rises and falls while following the camshaft, but because there is no pressure within the lifter, the piston within the lifter just floats, holding the pushrod in position while lifter rises and falls around it.
All 8 valves are closed and the pistons operate in a pressure/vacuum environment.
Beyond the parasitic drag of rings and bearings the pistons neither contribute to nor cost any measurable power.
At this time the injectors are also shut down and the coils are switched off.
So much for MDS...
Carbureted engines produced huge amounts of unburned hydrocarbon emissions (unburned fuel) because when you took your foot off the throttle and coasted, the vacuum below the closed throttle plate pulled fuel through the idle circuit.
Because the throttle was closed there was not enouph air below the throttle plate to support combustion.
As a result, unburned fuel just passed through the combustion chambers and out the tailpipe.
The worst examples were vehicles with manual transmissions because the RPM could not drop due to the fact the car was now turning the engine, through the clutch. But the automatics were almost as bad as they kept the engine spinning as well.
In the late 60's manufacturers introduced a method to inject a predetermined amount of air into the exaust manifolds, helping to finish burning the unburned fuel being pumped out through the exaust manifolds.
Old car guys know this as a Smog Pump.
It was nothing more than a belt driven low pressure high volume air pump using a diverter valve to meter air into the system and turned out to be quite effective at reducing unburned hydrocarbons throughout the RPM range.
Because it looked like it might cost horsepower....we threw them out by the trainload.
Anyways, control of unburned hydrocarbons at coast has been a engineering nightmare from the time we knew they were a problem.
Electronic fuel injection has just about turned this into a non concern.
Even though the vehicle is turning the engine at speed during coast (foot off throttle...say 60 MPH for example) and the throttle plate is closed, the ECM has shut the injectors off and the engine is nothing more than a big vacuum pump.
As long as the front pump within the trans is turning, it will transfer rotation from the wheels back through the driveshafts, trans, torque converter, and ultimatly the crankshaft.
If you just have to experiment, find a vacant section of road (fat chance) run up to about 60, release throttle and turn key to off. Do not take it out of gear.
Your going to have to guess your speed cause the dash will be dead during this experiment, but at about 30 mph turn the key back to ON (not start).
I haven't done it with my Jeep because....I'm not that inquisitive, but, you likely will find the engine is now running.
You have now manually done what the vehicle does electronically....you shut off the injectors during deceleration and restored the injector operation just as the front pump was loosing its ability to drive the engine due to lowering road speed.
So all that aforementioned B/S that preceeds boils down to is...YES the injectors shut off for a period of time during deceleration.
Rob
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Rob, as always, that was great- even better than those guys at Car & Driver were able to do. I think you should do one of those "trust me, I'm a doctor" TV spots for Dr. Pepper! I am finally convinced.
And when I go to sell the Jeep I can advertise it as a fuel efecient, MDS vehicle- when you step on the gas you use all the cylinders, and when you let off you use none! As we've seen earlier in this thread, our Jeeps can get up to 99.9 MPG highway!
And when I go to sell the Jeep I can advertise it as a fuel efecient, MDS vehicle- when you step on the gas you use all the cylinders, and when you let off you use none! As we've seen earlier in this thread, our Jeeps can get up to 99.9 MPG highway!
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... Back to the topic. 29.5 MPG this last week including a 250mile round trip at ~70mph on the motorway.
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Well Robby, Bet my (cheap?) diesel still costs a heck of a lot more than your (expensive?) gas. 
Never mind. Still a great vehicle either way. I posted the comment above as when I was figuring out what to buy there was very little info on the diesel. Hoefully someone may find my post useful.
Never mind. Still a great vehicle either way. I posted the comment above as when I was figuring out what to buy there was very little info on the diesel. Hoefully someone may find my post useful.
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Not me...and I've been getting 23mpg recently!
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I think you missed something. He was talking about the price you pay for gas (per gallon or litre).Not me...and I've been getting 23mpg recently!
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Yep I know!
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I finally got my new Commander and put 3898 miles on it in the first 2 weeks. Drove it around CA and then from Joshua Tree, CA to Rhode Island visiting friends/family and other places of interest (Arches National Park in Moab) along the way. I averaged 17.61 MPG on the total trip with as low as 16 on tanks with some offroading and as high as 20.5 coming across Kansas at 70 MPH. The Jeep was FULLY loaded with wife, 3 kids, and as much luggage and personal belongings that we could fit in the back and under our feet. I generally drive about 5 mph over the speed limit or with the flow (whichever is higher) on the highways.
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