Cylinder Deactivation......

[korax123]

I know there are other topics on this, but here is my idea. How come V8's and V6's have cylinder deactivation when cruising but no 4cyl car do. They can easily cruise off of 2cyl. I think it would save a descent amount of fuel on the freeway.

Since gas prices are getting higher some car companies should look into this.

[spacepilot]

Originally Posted by korax123

I know there are other topics on this, but here is my idea. How come V8's and V6's have cylinder deactivation when cruising but no 4cyl car do. They can easily cruise off of 2cyl. I think it would save a descent amount of fuel on the freeway.

Since gas prices are getting higher some car companies should look into this.

I think it's a wonderful idea. I don't know if it could be implemented as seamless as the 6- and 8-cylinder cases, though. IIRC, for a V6 with variable displacement, it operates with no less then 3 cylinder. For an I4, if there are only 2 cylinder working, the vibration might be an issue, and the driver may notice when half the cylinders are deactivated. I'm not an expert on engines, so I'm not sure if my concern is valid or not. But I agree, the manufacturers should look into this if they are not doing so. On the other hand, they probably would rather put more effort on pushing the hybrid vehicles, which most likely has a higher profit margin than plane-Jane 4-cylinder compact cars.

In fact, we could even take your idea a step further, so that all 4 cylinders can be deactivated. This could make coasting an ultimate fuel-saving technique. At present, when we coasting down in gear, we are using engine brake, although the engine is not using fuel, it is converting kinetic energy into thermal energy, which means that the fuel we used to accelerate is wasted. If we coast in neutral, the engine uses fuel to maintain idle speed. To save the most fuel, we could shut the engine off. However, this could be dangerous because power steering, power brake, ABS, airbags and other safety features may be deactivated. But if all 4 cylinders are deactivated, when we coast, no fuel is injected into the engine, we are not using engine braking, and all the safety features are still active. This is a win-win-win situation. One caveat is that either the car has to recognize when we want to employ engine brake (long steep downhill), and when we want to deactivate all the cylinders (coasting), or provide the driver a button so he can control the deactivation of the cylinders himself. Either way, it's more cost and liability for the car manufacturer with a benefit that only sensible drivers can reap. Seeing how most people drive on the road, I doubt engines with completely deactivateable cylinders will appear in passenger cars in the near future.

[korax123]

Originally Posted by theclencher

They have been. NVH is the problem.

Whats NVH?

And vibration wouldn't be a huge problem because there are tons of 2 cyl motorcycles and if they can cut a lot of vibes out of them. Then it is possible on a car spinning at fewer rpms.

[Lug_Nut]

Originally Posted by spacepilot

To save the most fuel, we could shut the engine off. However, this could be dangerous because power steering, power brake, ABS, airbags and other safety features may be deactivated.

1) the car is moving. There is much less need for steering assist with rolling wheels than stationary in a parking space. I recognize the issue of resuming assist and a sudden decrease in effort from low to almost none which could be an issue if in a turn when the steering pump resumes.
2) the vacuum reservoir maintains assist. Vacuum assisted brakes had a chamber that holds vacuum for days with no use, or for two, maybe three or more applications within a few minutes of engine stop.
3) the battery maintains electric power. The ABS uses electricity to rapidly shift the valves to apply / release the pedal applied hydraulic pressure in the brake system.
4) the battery maintains electric power. The airbag systems are electrically triggered.
Stopping the engine and maintaining electrical systems is not inherently dangerous.

[Pete]

Originally Posted by korax123

Whats NVH?

Short hand for Noise , Vibration and Harshness.

By the way the best option for a 4 cyl de activating to 2 cylinders is a flat four engine (Subaru , Volkswagen air cooled , Citroen air cooled etc.).

Cheers , Pete.

[spacepilot]

Originally Posted by Lug_Nut

1) the car is moving. There is much less need for steering assist with rolling wheels than stationary in a parking space. I recognize the issue of resuming assist and a sudden decrease in effort from low to almost none which could be an issue if in a turn when the steering pump resumes.
2) the vacuum reservoir maintains assist. Vacuum assisted brakes had a chamber that holds vacuum for days with no use, or for two, maybe three or more applications within a few minutes of engine stop.
3) the battery maintains electric power. The ABS uses electricity to rapidly shift the valves to apply / release the pedal applied hydraulic pressure in the brake system.
4) the battery maintains electric power. The airbag systems are electrically triggered.
Stopping the engine and maintaining electrical systems is not inherently dangerous.

Good point. So the ABS and airbags works when the ignition is in the "Accessories" position. I didn't know that, but it makes sense.

[Snax]

The vacuum assist and power steering issues are very simple to solve if the idea of reduced safety is a concern.

Vacuum assist can easily be generated electrically. Just plug up the manifold vacuum line and install an electric pump. EVers have been doing this for decades.

Power steering pumping can also be handled electrically by borrowing a pump from another car such as a second generation MR2. They work great and have the added benefit of being able to be modulated for more or less assist based upon speed.

[Pete]

Some alternators also have a vacuum pump fitted to them when they are bolted to diesel engines.
This may be an option.

Cheers , Pete.

[civic lover]

I just want to put my 2 cents in... 3 years ago a valve in my civic broke so I was down to 3 cylinders. The thing had absolutly NO! power. I could barely limp to my next deliver, and then home. Granted I lost compression in that cylinder and that may have added to the loss of power, but in any case I don't believe there is any room for shutting down cylinders.

[Snax]

No matter how you look at it, I seriously doubt it's viability as a fuel saving measure without some sort of valve control scheme. Unless your vehicle is grossly overpowered and undergeared on the highway, the working cylinders just have to work twice as hard.

[RoadWarrior]

I understand that for best efficiency, valve closing is desireable, but in looking into this for a while, I am wondering if it is actually 100% necessary for some savings, or more of a somewhat historical convenience dating back to manufacturers DOD efforts from the late 70s and early 80s... just the way they have done it...

In the beginning... manufacturers were experimenting with this on carburated engines... it's real hard to stop fuel getting into a cylinder from a carb without closing the valve. TBI motors have the same problems. Then when MPFI came along, most of the experimentation with valve closing had been done, it was some piece of technology they had sitting on the shelf as it were, too expensive to implement, and too complex to mechanically control satisfactorily within customers NVH expectations. As ECUs got more sophisticated, it became more of a practical proposition to put it under electronic control.

Bear in mind that while even fairly crude DOD schemes were quite effective at a steady state cruise, none of them have performed very well on the EPA highway cycle test cycle, due to very little of it being steady cruising. Ergo the cost/benefit numbers didn't appeal to the bean counters.

Another thing to remember is that Detroit knows of a far better Displacement on Demand scheme, it's been trying to get customers to accept it fully since the 80s, it's where you start out with a small motor, and give it the ability to burn as much charge and hence produce as much power as a large motor, it's called turbocharging. So if you've got a V8 windsor in a ford truck, you can do it this way, get the 4cyl mustang turbo motor in there instead.

Mainly with an MPFI car, the real obstacle to getting DOD working acceptably enough for an MPGgeek is the vacuum and fuel getting out of shape due to airflow metering, whether by MAF or MAP being wrong, or the O2 reading lean and making the ECU dump fuel. These problems might be worked around in a way that a manufacturer would consider a poor production solution when they have the ability to make mechanical and other changes for a valve closing scheme, which would avoid these issues.

Manufacturers also have to plan for "worst case scenario" type situations, where for instance the evap canister dumps fumes into the manifold at the same time the PCV is blowing in a lot of fumes and oil mist... if allowed into a cylinder not receiving fuel this could be enough to detonate lean, possibly causing damage, also minor injector leakage could have this effect. Now if only 10% of their customers mistreat their engines enough for this event to happen during the warranty period, this would be a heck of a lot of engines they would have to replace.

Anyway, I'm just trying to point out that there is a lots of room to play underneath the holy plateau of what is a perfectly engineered, marketable and profitable consumer product.

However... DOD isn't for all engines... yes some 4 cylinders may be a little too weak for it to be worth the effort and have inherent balance problems before you start deactivating anything. If the vehicle isn't a little overpowered, then the motor will have way too much work to do on 2 cyl , and you'll have the gas to the floor to keep it moving. I would regard DOD as a final stage modification on many vehicles rather than something to do to a stock 4cyl economy vehicle. First the vehicle needs to be easily driven at highway speeds, work on the aero, next the engine needs to be efficient and have some power in hand, properly sized and wrapped headers, crank scrapers, porting, are some of the modifications that might be desireable, 80% of performance modifications are efficiency modifications. So now if the motor is making about 130 or 140 HP where the base engine made 90, and is doing so efficiently, we might be able to think about screwing with DOD on it. The reason most people think performance mods kill MPG is because they can't keep their foot off the pedal.

Bear in mind that economy oriented 5th gear ratios will likely not be friendly to DOD, if the 4cyl motor is turning 1500 on the highway, it's close to lugging as it is, autos, or manual transmissions without available ratio swaps, might fare better when they're turning 2500-3500 RPM on the highway. In other words, if you have a car that's only acceptably powered with a 4cyl, if there's a way to get your gear ratios more favourable for highway cruising, do it instead, it's far simpler.

I have DOD plans for my minivan, I am inspired in this by knowing of a guy with the same van who has got 40mpg doing it. More on that another time.

Road Warrior

[RoadWarrior]

In looking into how O2 sensors work, I've just realised that the situation with crude DoD by injector deactivation and the O2 sensor reading lean may not be as bad as it seems like. The pressure in the exhaust system where the O2 sensor is, will be lower when there are cylinders not firing, even though they are pumping air through. Therefore the partial pressure of the O2 in the exhaust is lower than if it had all cylinders firing lean and putting that much O2 in. This makes the O2 in the exhaust stream influence the sensor cell less, so it doesn't read as lean as it is... from reports of folks who have tried this, it appears to be just enough such that the O2 isn't faulted as dead by the ECU, but it's likely that it does cause some enrichment, though one might say this is necessary for making enough extra power for the live cylinders to haul the vehicle along...

[spacepilot]

Originally Posted by RoadWarrior

The pressure in the exhaust system where the O2 sensor is, will be lower when there are cylinders not firing, even though they are pumping air through.

That's probably why proper variable displacement is not easy to DIY because it requires the intake and exhaust valves to be kept closed.