Merging of Gasoline and Diesel Engine Technology

After returning from NACAT 2007 in Long Beach, CA, I could not help but reflect on how the technologies we traditionally view as Gasoline/Automotive and those as Diesel/Truck are merging. In the recent past, we have seen a significant increase in direct injected (DI), gasoline-fueled engines – accompanied by increases in compression ratios that DI makes possible. DI gasoline fueling in conjunction with fast-response, Piezo actuated injectors also enables the engine controller improved control of cylinder combustion using multi-pulse injection events, a management strategy now used for a number of years in diesel engines.

In similar ways, diesel technology has borrowed from its gasoline-fueled counterparts. With an ever-increasing number of diesel engines adopting common rail fueling and the almost universal use of EGR, PCV, diesel particulate filters (DPFs), oxidation and reduction catalysts on diesels, post-2007 diesels may be cleaner at the tailpipe but are anything but clean in appearance. All this additional hardware and the array of sensors and actuators required to monitor and manage it, have contributed to making the diesel a cozier cousin of the gasoline-fueled engine.

2007 fuel systems

We have seen enormous advances in highway diesel engines for the EPA 2007 model year and certainly some of these changes have increased the commonality shared by diesel and gasoline engines. Highway diesel engines certified for 2007 now share just four fuel systems. The 2007 fuel systems are:
•   Common rail with electro-hydraulic injectors
•   EUI (electronic unit injection) with integral electro-hydraulic nozzles
•   EUP (electronic unit pump) with electro-hydraulic injectors
•   Cummins Time-Pressure (TP) injection
Even more surprising is that the last two systems in the above list are scheduled for replacement by 2010. All this promises to make life much easier for the diesel technician who in past years had to be familiar with a dozen distinct fuel systems.

Common rail

Diesel common rail (CR) fuel systems make diesel fuel injection almost as simple as standard gasoline multi-port fuel injection systems. Sure, the pressures run higher and they have a wide window of pressure variability controlled by ECM algorithm. But the electro-hydraulic injectors used by CR systems are essentially on-off valves. When switched by a PWM signal they open, injecting fuel at whatever pressure the rail is being managed at. Gone are the opening and closing lag times that were a feature of older hydraulic multi-orifice nozzles. If the CR rail pressure is managed at 20,000 psi then that is the pressure you get at the CR electro-hydraulic nozzle – almost instantly. And because the specific rail pressure at any given moment of operation translates into a defined emitted droplet size, CR enables the ECM to precisely manage cylinder combustion. Add to this multi-pulse injection events and the result is near perfect control over cylinder pressure. Both solenoid and piezo actuators are used with diesel CR. Both offer multi-pulse capability. But piezo actuators (they function on the principle of the reversibility of piezoelectricity) are better suited to the super-fast responses required when up to 7-separate injection events are required in one cycle.

EUI with electro-hydraulic nozzles

If anyone had suggested that Caterpillar, Detroit Diesel, and Volvo/Mack would be using the same injectors a few years ago, you would be forgiven for being a little skeptical. But exactly that has resulted in each OEM’s post-2007 EUI-fueled engine families. The Delphi E3 injector is essentially a solenoid cartridge controlled EUI used by the diesel industry for two decades – with one essential difference. The hydraulic nozzle valve that provided fixed-pressure opening and closing is gone. In its place is an electro-hydraulic nozzle that provides the EUI with many of the advantages of CR systems: ‘soft’ that is, ECM-controlled nozzle opening pressures and abrupt closure of the nozzle valve at the conclusion of injection. The eliminating of the collapse phase at the end of injection that disadvantaged hydraulic nozzles was a key to readying this new generation of EUIs to meet 2007 emissions. The Mercedes Benz EUP-fueled engines have used a similar 2007 strategy: the cam-actuated EUP remains but the previous generation hydraulic nozzles are replaced with ECM-controlled electro-hydraulic nozzles.

The future?

The marriage of Detroit Diesel and Mercedes Benz set in motion plans for a common platform engine, reportedly due slightly ahead of the U.S. 2010 EPA year. As yet there has been no definitive information on how this new engine will be badged. Will Detroit Diesel even survive as a brand? However, it has been stated that the new common platform engine will adopt CR-fueling. This, combined with Cummins’ announcement that they will abandon the TP fuel system used on their flagship ISX engine for CR, indicates that all highway diesel engines will be fueled by just two systems by 2010. This might translate into good news for students of diesel technology. The study of diesel fuel systems has traditionally been one of the tougher college courses and it promises to become one of the easiest!

Sean