WHY A DIESEL?
by Robert Patton, TDR Editor
Back in Issue 55 we published the entire “Why a Diesel” reprint from an earlier 1996 TDR magazine. Realizing that there is always readership turnover, it is tempting to do another full-blown four page reprint. Yet, for those long-time TDR members, it would be wasted space.
A compromise is in order.
This brief excerpt from the “Why a Diesel” article takes you to the reason for the diesel engine’s greater efficiency — the fuel injection pump and the ability to vary the air-to-fuel ratio corresponding to the load on the engine. The following is the reprint:
Rudolf Diesel designed the heat engine concept to use the injection of fuel at the last moment to ignite the hot compressed air. Understanding the heart of the diesel, the fuel injection pump, is the key to answering the fuel efficiency question.
The Gasoline Engine
A gasoline engine is what engineers call “stochiometric.” Stochiometric describes the quantitative relationship between two or mor substances, especially in processes involving physical or chemical change. With a gasoline engine there is a stochiometric equation of 14 parts of air to one part of fuel. Remember, always 14:1. Whether at idle or full throttle, the fuel and air are mixed outside* the cylinders in a carburetor or injection manifold, and the mixture is introduced to the combustion chamber via the intake valve, 14:1, always.
*Exception: New direct injection gasoline engine’s that are now being offered from at least six different vehicle/engine manufacturers.
The Diesel Engine
Fuel and air in the diesel design are not premixed outside the cylinder. Air is taken into the cylinder through the intake valve and compressed to make heat. Diesel fuel is injected near the top of the piston’s stroke in an amount or ratio corresponding to the load on the engine. At idle, the air-to-fuel ratio can be as high as 85:1 or 100:1. At full load, the diesel still boasts a miserly 25:1 or 30:1 ratio! It is in the injection system where we find the key to the diesel’s fuel mileage superiority.
The Fuel Pump is the Key
The fuel pump used on early ‘90s vintage diesel pickup trucks typically was a rotary style fuel pump. Think of this pump as a mini automobile-spark-distributor. A rotary head sends fuel pulses through the high-pressure fuel lines to the injectors. The pressure opens the injector valve, and fuel is injected.
As exhaust emissions standards tightened in 1994, there was a need for higher fuel injection pressures and more timely delivery of fuel into the combustion chamber. Pickup truck leader, Ford, used an injection system developed by Caterpillar called HEUI (hydraulicallyactuated, electronically controlled, unit injection). The Dodge/Cummins engine used a Bosch P7100 in-line fuel pump. Think of it as a mini in-line six cylinder engine, and it’s easy to understand its principle of operation. Six plunger pumps actuated by the pump camshaft send fuel pulses through six high pressure fuel lines to the injectors. The pressure opens the injector valve, allowing fuel to pass into the combustion chamber. With the Bosch P7100 fuel pump the metering of the fuel (at idle, 85:1; or at full load, 25:1) is controlled by a fuel rack and gears that rotate a metering helix to allow fuel into the six plunger pumps.
Future Considerations
Further exhaust emission legislation in 1998 and again in 2002 has forced the diesel engine manufacturers to introduce electronic fuel injection controls. Key legislation dates were 1988, 1994, 1998, 2002 and 2007. Thus, the progression from simple mechanical (vintage 1988-1993) to more complex mechanical (vintage 1994-1997) followed by simple electronics (vintage 1998-2001) and now advanced electronics (2002 and newer) has been the norm that the diesel industry has followed.
The January 1, 2007, emissions legislation brought another dramatic decrease in exhaust emissions for diesel engines in pickups and big-rigs. The Cummins engine’s displacement was increased to 6.7-liters. Exhaust gas recirculation was added to the engine hardware. Aftertreatment components were added to the exhaust system. The engine continued with the Bosch high pressure, common rail (HPCR) fuel system as was used on the model year ’03 to early ’07 5.9-liter engines.
Year 2010 will see, yet again, another round of tightened emission legislation. The good news for Dodge/Cummins owners is that the 6.7-liter engine that was introduced to meet the January 01, 2007, standards has already been certified to meet the 2010 legislation.
Thus, product continuity until the next emissions hurdle. (2013? See BITW, page 46.)
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