It's off the subject of my 64 Rdst but now that GM has followed Chrysler with Variable displacement i am very curious and hope some of the engineering types out there can help.

I'm wondering just how the hemi goes so effortlessly from an 8 cyl. to a 4? Is it electrical, do the pistons continue to go up and down and still cause drag? Anyone know the engineering used here?

regards

 

The Chrysler MDS (multi-displacement system) on the Hemi works by interrupting the oil pressure to the lifters of the four cylinders that are deactivated; the interrupting valve is controlled by the engine control computer. The valves in those four cylinders remain closed until the interrupter valve opens up and allows oil pressure to the lifters again.

 

I believe the GM system works in the same manner as Chrysler's, using a solenoid to provide or deny oil pressure to special lifters.

Honda has a different system of variable displacement that is based off their VTEC system. VTEC uses multiple cam lobes of different profiles for each valve, with rocker arms following each lobe. One rocker arm, or a separate arm, pushed on the valve. A hydraulicly actuated pin locks rocker arms to the valve arm to select different cam lobes. In Honda's variable displacement system, the pin can also completely disconnect the rocker arms from the valve arm, leaving the valves closed.

It's nifty stuff, but I have to wonder at just how much energy is lost in compressing and decompressing the trapped gases. I know the compressed gas's pressure returns a lot of the compression energy back to the crankshaft on decompression, but it can't be perfect return.

Cheers,
-Brian

 

John - Wouldn't the exhaust value have to remain open? If both valves are closed it would create too much resistance (compression with no place for it to go) on the up stroke, and too much vacuum on the down stroke

 

No, the exhaust valve does not stay open (it stays closed). What they found was that the piston acts as a spring with the valves closed off. The compression generated on the upward motion of the pistion forces the piston back down on the downward motion, canceling out the power lost during compression. Also of course, the injectors for those deactivated cylinders are turned off as well!

I have owned a Dodge Magnum Hemi since Sept. 05. It is a great engine (and is a neat vehicle overall), but does not live up to the EPA fuel estimates for how I drive it. Best I have seen is 23 on the highway (cruising at 70+), and local is around 19. I have the fuel economy readout, and it shows a great difference when the MSD kicks in. I added a separate LED to tell me when the MSD is activated, and that helps my lead foot condition. Today after a fill up, I saw 40+ MPG while cruising at 40 (MSD activated). Too bad you can't do that all the time!

Plasticman

 

The GM system only deactivates the inlet valves. A solenoid pulls a pin in the lifter that prevents cam action from being transmitted to the pushrod, and, of course, the injectors are simultaneously disabled.

Two inside cylinders from one bank and the two outside cylinders from the opposite bank are deactivated. This leaves four cylinders operating at even 180 degree firing intervals, like an inline four, but without the unbalanced second order vertical shaking force.

The exhaust valves continue to open and close, which helps maintain cylinder temperature and even temperature gradients throughout the engine.

The combination of modern electronics and better hardware makes the transistion seemless to most drivers. Don't bring up the Cadillac V-8-6-4 system from 25 years ago. These new systems are worlds ahead.

GM announced this "Displacement on Demand" system over two years ago, including good schematics and explanation of its operation, but I think maybe Chrysler actually beat them to the market with their cylinder deactivation system. GM said that EPA mileage numbers would increase about 15-20 percent with DoD with the biggest improvement at sustained steady speed, like highway driving.

When running on four cylinders, the pumping loss from the deactivated cylinders is eliminated and reduced on the remaining four because more throttle (less manifold vacuum) is required to maintain road load power. Pulling the cylinder down against the inlet vacuum is not free. It's a major internal parasitic load, which increases fuel consumption. That's one reason why diesels are more effeicient, especially in stop and go driving. Since diesels aren't throttled they have very low pumping loses.


Duke

 

Wow, you guys are great. I have a beloved 05 300C and I've always wondered how the dam thing works. My milage is about 20---22 MPG but honestly driving the car is such a rush that my foot is always into it; at least when my wife lets me take it.

Thanks again