The cylinder head of a DI engine has a flat deck with no combustion chamber. The valves are flush with the cylinder head. Our example is from a 7.3L Ford Power Stroke.
Indirect vs. Direct Injection
In diesel engines, the utilization factor (the ability to have smoke-free combustion) varies with the combustion system. For example, the diesel engine featuring a precombusiton chamber system, or indirect injection (IDI), has a utilization value of about 80 percent. The earlier naturally aspirated direct-injection system has a value of about 70-80 percent. The turbocharged direct-injection (DI) system has values of approximately 50-70 percent.
The utilization percentage of light-duty diesel engines with direct injection has been improved upon greatly with recent designs. This is the result of advanced combustion chamber technology and electronic controls along with the need to limit tailpipe emissions.
A DI engine has the combustion chamber cavity in the crown of the piston. This design is called a Mexican Hat piston. Note the size of the pin bore for the wristpin. You will not find anything like that in a gas engine!
An IDI system sprays the fuel into a small precombustion chamber where the ignition occurs and then spreads out into the main combustion chamber. In contrast, a DI engine has no prechamber. Due to the differences in pressure rise, combustion chamber shape, and burn speed with an IDI versus a DI design, the combustion sound becomes unique. To a trained diesel enthusiast, each combustion chamber will produce a unique noise.
Interestingly, the latest version of the GM Duramax has a slightly lower compression ratio to limit the noise generation of this already extremely quiet engine.
The compression ratio of a diesel engine is usually indicative of the combustion chamber design. Due to an increased surface-to-volume ratio and its inherent negative impact on thermal efficiency, an IDI design will employ a higher static compression ratio of usually 20:1 or more. In contrast, a DI diesel has less combustion chamber surface volume, so a compression ratio of approximately 15:1 to 17:1 is sufficient. In addition, an IDI application would depend more on forced induction to clean up the exhaust over a DI engine.
Why Diesels Don't Make Horsepower
Since any Otto cycle engine's output is dependent on the amount of air the pistons can pump, high rpm are required to produce horsepower. The higher the rpm, the greater the potential for horsepower. This is due, in part, to the definition of horsepower being work over time.
GM light-duty diesel engines prior to the Duramax employed either a Stanadyne (shown) or Roosamaster rotary injection pump. The example is from a 6.5L GM engine.
The quicker the engine can do the work, the more horsepower it has. A naturally aspirated diesel is limited in power per cubic inch simply because the burn rate of diesel fuel is very slow when compared to gasoline. Thus, by nature of the chemical composition of the fuel, the diesel engine is slow to rev and has limited maximum rpm. For this reason, the diesel engine responds favorably to forced induction through turbocharging or supercharging.
Turbocharging a Diesel
Since the mid 1960s, the heavy-duty use of diesel engines was increasingly offered in turbocharged form. This was originally a means of improving power output in the medium- to high-speed range, but recently, especially with the light-vehicle market, turbocharging has been employed to produce an environmentally cleaner engine. The turbocharger increases the amount of air delivered to the combustion chamber so the injected fuel can be burned more efficiently. The benefits are a lower fuel consumption rate for the power developed, and as a consequence, better control of emissions. Furthermore, the engine will be quieter because the turbocharger has a silencing effect on the induction and exhaust noise.
The '94-'9811/42 Dodge Cummins used an inline Bosch injector pump. The injector pump creates the fuel pressure necessary to open the mechanical injectors. This known as pop-off pressure.
Forced induction increases the volumetric efficiency (VE) of the cylinders not through piston velocity but with an external fan. VE describes the amount of the cylinder that is filled with charge. Contrary to what many believe, on any normally aspirated engine (regardless of the fuel consumed), the cylinder bore is never completely filled with charge, which describes fuel mixed with air. A production gasoline engine will experience approximately 80 percent VE at peak torque while a normally aspirated diesel, due to the low piston velocity, may experience only 55-60 percent VE.
Forced induction when applied to a diesel engine makes up for many of the disadvantages of the slow-burning fuel. Since a diesel does not experience detonation in the same manner as a gasoline engine, both a high compression ratio and large amounts of boost pressure can be employed simultaneously with excellent results.