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30 MPG w/ the 4.0

9

93_xj

NAXJA Forum User
Location
Hampton, VA
How its done:

My buddy whos going to school in ohio to be a deisel mechanic. His teacher drives an 03 wrangler with a 4.0 and was able to get some pretty significant gas mileage numbers with it. He claims that he gets 30 mpg on the highway.

What he did was put 5 pin relays on cylinders 2 and 5 (because they are running partners when one is firing the other is exhausting) and wired them so that when he set his cruise control (also can be done with switch) and his torque converter went into lock-up, his injectors "shut off" making his 6 cylinder act as a 4. He then went ahead and added a resistor with the same resistance as a running injector to make the computer think they were still spraying.

Diag

WiringDiagram.jpg



 
Some propane (LPG) systems use a component called an 'Injector Emulator' to do the same thing as the resistor in the diagram. Perhaps, these could be used instead if they are easier to find? They may even have the relay built in.
 
I would definetly be interested in this. Although I wonder if it may not be that beneficial with heavier rigs that have a lift, bigger tires, bumpers, etc. It may bog down the "4 cylinder" too much.
 
This may sound kind of dumb but here goes, OK you turn off the injectors so that no gas is put into the cylinders, don't the cylinders have to do the same amount of work on the compression stroke compressing now an air charge ?
Like it is now working a 2 cylinder compressor and the other part of the question is now you are also sending a mixture rich in air in to the exhaust stream which has to have an effect on the O2 sensors.
 
RichP said:
This may sound kind of dumb but here goes, OK you turn off the injectors so that no gas is put into the cylinders, don't the cylinders have to do the same amount of work on the compression stroke compressing now an air charge ?
Like it is now working a 2 cylinder compressor and the other part of the question is now you are also sending a mixture rich in air in to the exhaust stream which has to have an effect on the O2 sensors.
Click to expand...

Very good point. I would think OBDII would see lean conditions from O2 sensors and richen fuel mixture up. How can that help MPG?
 
RichP said:
This may sound kind of dumb but here goes, OK you turn off the injectors so that no gas is put into the cylinders, don't the cylinders have to do the same amount of work on the compression stroke compressing now an air charge ?
Like it is now working a 2 cylinder compressor and the other part of the question is now you are also sending a mixture rich in air in to the exhaust stream which has to have an effect on the O2 sensors.
Click to expand...

My question would is? What happens to the spark plug? Is the spark to those cylinders turned off too? Would a lean condition cause the cylinder to heat up and melt down? I think of it as plug readig to detect a rich or lean condition that would have detremental effect on the engine. Is the 4.0 RPM's too low that it would not matter?
 
Someone, maybe me, needs to look in to how it is done in the GM cars. I had some one tell me that they were multi-cam cars. The computer controlled the cams, much like Vtec, and when the conditions were met for the shut off, the cam for that set of cylinders would open the exhaust valve so no compression would happen. I dont know how accurate that is but it sounds good. As everyone else has said, I think there is a lot more that goes into it than simply turning off the injectors and telling the computer they are still firing...
 
GM completely closes both valves. They do it in the pushrod motors, by a special lifter.

In support of cylinder deactivation is some very interesting choreography from things ranging from throttle valve modulation to active exhaust tuning, but it all starts with the additional job tasked to the lifters. "We disable the valves through a device called a switching lifter," explains Meagher. "This differs from a normal lifter in that there is an inner body and an outer body connected by a spring-loaded pin. For V-8 operation, the pin is fully expanded by the spring so the two pieces act as one and the lifter acts like a regular lifter. When we want to disable the valve operation, we deliver high-pressure oil to a groove in the lifter that leads to the outside end of the pin, forcing the pin to collapse the spring. Now the two parts of the lifter are free to move relative to one another and as the cam lobe pushes on the follower the inner portion of the lifter pushes against another spring at the top of the lifter and does not transfer force to the pushrod."
Click to expand...

And yeah, you nailed it, unless you eleminate the pumping losses, you really result in no net gain, by just shutting off the injectors. It takes X ammount of fuel to create Z ammount of energy, at Y% effeciency. So, unless you either increase effeciency, or decress the ammount of energy needed, you still need the same ammount of fuel.
 
The injector on those two cylinders wouldn't spray. If you take power from them there not going to spray anything. Now the two dead cylinders would act as a "compressor" tho, not sure how that would be with helping gas mileage.
 
I don't have any specific details, but the Northstar system in the Caddy's had something similar to this I believe. It's been awhile, but I believe it would shut down two cylinders to pump air for cooling in case of overheating.
 
Pretty sure Dodge has this setup too...
 
jeeperzcreeperz said:
The injector on those two cylinders wouldn't spray. If you take power from them there not going to spray anything. Now the two dead cylinders would act as a "compressor" tho, not sure how that would be with helping gas mileage.
Click to expand...

It's honestly not going to take much of the engines power to simply spin those dead cylinders, especially because the 4.0L has a realitively low compression ratio.

The main thing here is that the rig is using far less fuel to reach a given power output. That is because most of the energy that results from combustion ends up being turned into heat, instead of being used to turn the engine. If you take away those two cylinders, you will reduce the amount of wasted energy. The remaining 4 cylinders are going to have to work harder to reach the amount of power required to maintain a certain speed, but that increase in work is still less than having those two extra cylinders working at the same time.

I hope that makes sense, but almost all car manufactures are producing certain engines like this right now. however we don't see more of them because of the transition to hybrids.

I actually had plans to do this exact same mod once I turbo charge my rig. I will run E85, and by cutting out those 2 cylinders I will be able to get a highway mileage that is about the same as regular 85 dino.
 
Cadillac used to have this in an engine called the 4-6-8 where it would use all eight cylinders when needed and drop to 4 when cruising, but being an early design it had a huge number of problems. The semi-hemi 5.7L (I think) has this feature now. The Cadillac Northstar does not use it for normal mode but it has a limp-home mode where it will cut out cylinders and switch between them if the engine gets into a really bad way, but that is only for use in emergency so that you aren't stranded in the middle of nowhere.
 
Stroketech said:
Interesting, for as big of a turd as the 4.0 is on the highway I can only imagine what it would be like with 2 cyl's shut down.
Click to expand...

Hey, I have no issues with my 2.5TJ except with hills and headwinds and 5th gear :D :D :D
 
There would be 2 bores worth of plain air being pumped into the exhaust...could this upset the 02 sensor? How about the rod and main bearing life now that the firing pulses aren't smooth to the crank? Sounds like trying to reinvent the wheel to me.
 
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