July 10, 2002

GM's Displacement on Demand
by Jim Kerr

It's a little like having your cake and eating it too. The technology is called Displacement on Demand, and it is set to provide a combination of power and fuel economy in GM's SUVs and trucks beginning in 2004. Increasing fuel economy by up to 25% in real world testing, the displacement on demand system saves fuel by using only half of the engine's cylinders during most normal driving conditions, and uses all the cylinders when accelerating or carrying loads.

This concept is not new. GM tried it 21 years ago. Some may remember the Cadillac 4-6-8 V8 engine introduced in 1981. This engine ran on 4,6 or 8 cylinders as loads demanded, but it was limited by a cable throttle, mechanically controlled transmission and a fraction of the computer power currently available. While the concept was fine, the technology wasn't. The Cadillac engine was rough and sometimes erratic when switching the number of operating cylinders. Most of these cars had their engines soon converted to work on eight cylinders all the time.

So what is different this time? Computing power, and lots of it. Today's engine computer is a 32-bit machine (versus 8 bit) that has an internal clock that is roughly 25 times faster, 50 times the computing power and 100 times the memory of the 1981 controller. Electronic Throttle Control and electronically controlled transmissions are now proven technologies that allow seamless Displacement-on-Demand operation. With the improved technology, engine torque control is immediate and continuous, so the driver's perception of engine operation is a smooth increase in power when needed and normal running when cylinders are deactivated.

While Displacement on Demand mechanical technology could be implemented on overhead camshaft and four-valve per cylinder engines, the most economical implementation is on pushrod operated two-valve per cylinder engines - just like the majority of GM's Vortec family of truck engines. The engine always starts on all cylinders, but once the vehicle is moving, the powertrain control module activates Displacement on Demand and deactivates cylinders if they are not required.

Operating the engine on half of the cylinders during light-load conditions enables those cylinders to achieve higher efficiency. The throttle is open more, so pumping losses are reduced because the pistons don't have to pull against as high an engine vacuum. Thermal efficiency is better because the operating cylinders are working more. Engine volumetric efficiency is improved.

Special hydraulic lifters are a key element of the system. Developed by Eaton Corp., the lifter is designed so that one section can collapse, or telescope, into the other section. The two sections can be either coupled or uncoupled to each other by means of a locking pin. When coupled, the lifter can transfer the lift of the camshaft to the rest of the valve train. When uncoupled, the lifter acts like a spring and the valve train doesn't move, stopping that cylinder from producing power.

Hydraulic oil pressure, supplied by the engine oil pump and controlled by computer-activated solenoids, is used to dislodge the locking pin and collapse the lifter, thus closing the valve. In reactivation mode, removing hydraulic pressure causes the locking pin to return to its latched position to restore the lifter's normal function. The powertrain control system monitors driver commands and several engine sensors to control spark advance, fuel injectors, electronic throttle and valve train control solenoids to provide a smooth transition between V4 and V8 operation. The reactivation of cylinders happens so quickly that there is immediate engine response to driver's power demands.

GM initially plans to produce more than 150,000 Displacement on Demand V8s in the first year. Production will then be increased to nearly 1.5 million units in 2007. With SUV's and full sized pickups able to achieve up to 160 km more on a tank of fuel, Displacement on demand will make a significant impact by reducing exhaust emissions, preserving our oil reserves, and perhaps most important to drivers, lowering fuel costs.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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