Cylinder Deactivation......
 

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.

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.

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.

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.

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.

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.

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

Cheers , Pete.

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.

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.