Interesting project

[poogie]

I would like to see someone perform an experiment of measuring the power loss of an engine missing on one cylinder. A 4 cylinder would have to lose more than 25% of its power if it was a constant miss because the other working cylinders would have to turn the engine against compression. of the dead cylinder. I would like to see this experiment performed on a dynometer at varying engine speeds. With an open exhaust valve be the cause if the miss would the power loss ne exactly 25% on a 4 cylinder engine? Has this experiment been performed in the past?

 

[Bob]

With an open valve, you would loose more from "pumping losses"... moving the air in and out of the cylinder than you would lose by keeping the exhaust valve CLOSED.

With the valve CLOSED, the air is compressed, then springs back. With the valve working, you are PUMPING air. (Think "Jake Brake.")

Take a look at how the Goat's 318 or the General's 5.3 "displacement on demand" engines operate... lifters are made to collapse under control of the 'puter, so the valves remain closed, and also fuel injection to the "shut down" cylinders is stopped.

 

[richardinnz]

Yes, its called a Morse test, using a dynamometer the B.E.M.P. of each cylinder can be calculated. Did it 30 years ago at college, using a brand new Massey-Ferguson 575 tractor, never done it since!

 

[RayP(MI)]

OK, so that brings up the question of some of the more exotic car engines (thinking Cadillac Northstar, but could be wrong), that are capable of running on 4 cylinders when under light load, rather than their full 8. How do they accomplish that?

 

[Sid]

QUESTION. Bob, at what point do the valves close? It seems to me they would need to close at mid stroke so as not to be working against full compression and not working against a vacum force.

 

[Sid]

I don't understand how freely moving air back and forth(valves open) as compared to working against closed valves looses more power than valves closed. I am thinking "Jake Brake", equals, Creating resistance.

 

[Bob]

On the variable displacement engines, the lifters are simply allowed to remain collapsed, so the valves don't operate. Air that's compressed in the cylinders "springs back" as the piston goes down, giving back most of the energy it took to compress it.

On a "Jake Brake", the exhaust valves are popped open at the height of compression "wasting" the air compressed in the cylinder, thereby providing braking action from the "pumping losses".

 

[Mike (WA)]

I had an '81 Cadillac 8-6-4. There were electromagnets built into the valve cover over the proper valves, so when it went into 4 or 6 cylinder mode, the valves were held open magnetically to as not to create back pressure. At the same time, the computer shut off the fuel to those cylinders. The system actually worked OK on mine, but apparently not on many others- it only lasted one year. Didn't really help much with gas mileage. I assume the system on the Northstar is more sophisticated, although I don't know how you'd hold the valves open except with the electro-magnets.

 

[Bob]

"With Displacement-on-Demand, the powertrain control module determines load conditions from vehicle sensors and driver commands. Under light load this sophisticated, 32-bit controller automatically closes both intake and exhaust valves on alternate cylinders of each cylinder bank (for example, numbers 1, 7, 4 and 6 cylinders). The valves are reopened the instant the control module determines that vehicle speed or load requires more power. The module also controls fuel injectors, electronic spark advance, and electronic throttle for transition between V-4 and V-8 operation so quickly that engine output increases immediately. The switchover is seamless and virtually imperceptible. The engine is started on eight cylinders."
Link

 

[Sid]

Thanx for the answer.

 

[Spook]

GM builds a lot of DOD engines. The DOD's we put in trucks are so popular that we can't build enough. The description below is excellent.