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6th July 1995, Page 40
6th July 1995
Page 40
Page 41
Page 40, 6th July 1995 — NJ CT 011 OFT
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Just when you thought that diesel technology couldn't go much further, Navistar has found a way of replacing camshafts with oil...

Afour-stroke engine without a camshaft or other mechanical valve actuation could be just around the corner. The hint of things to come was the production launch of Navistar's straight-six 7.6-litre DT466 and 8.7-litre 530 International engines, rated at 250 and 300hp (186/224kW) respectively, with 660 and 10501bft (895/1,424Nm) of torque.

Both engines are available with conventional mechanical injection, but they also have the option of hydraulically actuated electronic unit injectors (HEM). Navistar expects 80% of the new units to be supplied with HEUI by the end of the year.

The HEUI system, developed as a Navistar/Caterpillar joint venture, works on three fluid loops: two of lubricating oil; one of fuel. From the normal oil pump most of the lubricant passes through the engine's bearings as usual. The rest passes to a Mannesman high-pressure pump. A comput

erised control valve sets the required pressurise to between 31 and 200bar (4503,000psi): pressurised oil is then fed to the top of the injectors. Any unused high-pressure oil returns to the sump (the second loop) while the injectors have the normal spill return (the third loop).

Inside the injector is what Navistar calls a "pressure amplifier". This is a hydraulic piston with one end seven times the surface area of the other. This means that any pressure on the larger end of the piston is multiplied by seven at the smaller end. It doesn't take a genius to work out that its the fuel that gets the pressure amplification.

Computer-controlled

Each injector has a computer-controlled solenoid valve which prevents the oil entering the chamber until the right moment, and shuts it off again when the correct amount has been injected.

This gives injection pressures of up to 1,400bar (21,000psi)—and it does it at all engine speeds. With rotary, in.line and cam operated unit injectors the injection pressures and duration (in degrees) may remain constant, but in absolute terms the injection rate will fall with engine speed. But with the Navistar/Caterpillar system full oil pressure is always available so the injection rate is always at its optimum, says the companies' engineers. Using Hall Navistar's engines are meeting the US 1998 emission regulations (circa Euro-3 predictions) with no need for after-treatment.

Navistar International's engineers say that the same technology will soon be harnessed to control the valve gear to manage the air flow into and exhaust out of the engine. In its ultimate form, this would include full electronic control of hydraulically actuated valves. Caterpillar and the Detroit car companies are working on parallel projects so we probably won't have to wait long.

A video of two trucks—one using a secret prototype of the hydraulic valve system on a 300hp International I-6 engine; the other with a similarly rated Cummins L10—shows the International-powered truck outperforming the other by a handy margin. The big difference, says Navistar chief engineer Pat Charbormeau, is the use of the HEUI injection system and the electronidhydraulic air management. Charbonneau will not go into detail about the controls on the prototype engine, but he does confirm that the company is working on electronic controls for the whole air management of a diesel engine.

The main obstacle to electronic operation of the valve train is the force required to overcome the combined load of valve spring tension and cylinder pressure on the exhaust valve. But Navistar engineers say that even with today's combustion chamber pressures, opening an exhaust valve takes only about half the equivalent pushrod force than that needed to generate these high fuel-injection pressures. The inlet valve is less of a problem as its opening is assisted by the vacuum created as the piston descends on the inlet stroke.

These forces have been difficult to overcome with electro-mechanical systems using conventional solenoids because the power developed by solenoids is relatively small. But with 300bar on hand HEUI has more than enough capacity to open and shut inlet and exhaust valves, again using solenoids simply to signal the timing. This opens up a huge and fertile field for development of engines that, for the first time ever, are unshackled from the restrictions of running mechanical camshafts Designers will no longer be hamstrung by fixed timing, fixed lift or by with valve opening and closing events limited by the acceleration of a roller on the face of a camshaft. Some demonstration power units have already been built.

Now the only limitation is the acceleration of the in-head valve and the hydraulic actuator: there's almost no restriction on valve lift or timing The freedom of valve operation from crankshaft angle is achieved by using parallel pentium computer chips. As the pentium chip is the latest thing in computing power, the use of two of them confirms Navistar's development direction. This massive computing power will be necessary to accommodate the quantum leap in engine controls implied by a design without a camshaft.

And when will the mechanically simplified engines be here? Far sooner than we might think, and certainly before the end of this century—a scant four years away.

by Steve Sturgess and Colin Sowman

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Organisations: US Federal Reserve
Locations: Detroit

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