Diesel or oil engines The defining characteristic of the diesel engine is that it relies on
compression ignition. As air is compressed it heats up. Fuel is then injected into the hot, compressed air and ignites spontaneously. This allows it to operate with a lean mixture comprising mainly air. Together with the high compression ratio, this makes it more economical than the petrol or gasoline
Otto engine. It also does not require either a
carburettor to mix the air and fuel before delivery, or a
spark plug or other ignition system. Another consequence is that to control speed and power output, the airflow is not throttled but only the amount of fuel injected at each cycle is varied.
Two-stroke cycle model of a
MAN B&W two-stroke
marine diesel engine with the
piston rod attached to a
crosshead In the two-stroke cycle, the four stages of internal combustion engine operation (intake, compression, ignition, exhaust) occur in one 360° revolution of the crank shaft, whereas in a four-stroke engine they take two complete revolutions. Consequently, in the two-stroke cycle the stages overlap through most of the engine's operation. This makes its
thermodynamic and
aerodynamic processes more complex. Because the four-stroke cylinder fires only every other revolution, the power output of the two-stroke cycle is theoretically twice as much. However, the scavenging losses make this advantage difficult to achieve in practice. •
Intake begins when the
piston is near the
bottom dead center (BDC). Air is admitted to the
cylinder through ports in the cylinder wall (there are no
intake valves). All two-stroke diesel engines require artificial aspiration to operate, and will either use a mechanically driven
blower or a
turbo-compressor to charge the cylinder with air. In the early phase of intake, the air charge is also used to force out any remaining combustion gases from the preceding power stroke, a process referred to as
scavenging. • As the piston rises, the intake charge of air is compressed. Near top dead center, fuel is injected, resulting in combustion due to the charge's extremely high pressure and heat created by compression, which drives the piston downward. As the piston moves downward in the cylinder, it will reach a point where the exhaust port is opened to expel the high-pressure combustion gasses. However, most current two-stroke diesel engines use top-mounted
poppet valves and
uniflow scavenging. Continued downward movement of the piston will expose the air intake ports in the cylinder wall, and the cycle will start again.
Two-stroke diesels In most
EMD and
GM (i.e.
Detroit Diesel) two-stroke engines, very few parameters are adjustable and all the remaining ones are fixed by the mechanical design of the engines. The scavenging ports are open from 45 degrees before BDC, to 45 degrees after BDC. However, some manufacturers make the scavenging port timing asymmetric by offsetting the crankshaft. The remaining, adjustable, parameters have to do with exhaust valve and injection timing (these two parameters are not necessarily symmetrical about TDC or, for that matter, BDC), they are established to maximize combustion gas exhaust and to maximize charge air intake. A single camshaft operates the poppet-type exhaust valves and the
Unit injector, using three lobes: two lobes for exhaust valves (either two valves on the smallest engines or four valves on the largest, and a third lobe for the unit injector). Specific to EMD two-stroke engines (
567,
645, and
710): • The power stroke begins at TDC ([0°]; injection of fuel leads TDC by 4° [356°], such that injection of fuel will be completed by TDC or very shortly thereafter; the fuel ignites as fast as it is injected), after the power stroke the exhaust valves are opened, thereby greatly reducing combustion gas pressure and temperature, and preparing the cylinder for scavenging, for a
power stroke duration of 103°. • Scavenging begins 32° later, at BDC–45° [135°], and ends at BDC+45° [225°], for a
scavenging duration of 90 degrees; the 32° delay in opening the scavenging ports (constraining the length of the power stroke), and the 16° delay after the scavenging ports are closed (thereby initiating the compression stroke), maximizes scavenging effectiveness, thereby maximizing engine power output, while minimizing engine fuel consumption. • Towards the end of scavenging, all products of combustion have been forced out of the cylinder, and only "charge air" remains (scavenging may be accomplished by Roots blowers, for charge air induction at slightly above ambient, or EMD's proprietary turbo-compressor, which acts as a blower during start-up and as a turbocharger under normal operational conditions, and for charge air induction at significantly above ambient, and which turbocharging provides a 50-percent maximum rated power increase over Roots-blown engines of the same displacement). • The compression stroke begins 16° later, at BDC+61° [241°], for a
compression stroke duration of 119°. • In
EFI-equipped engines, the electronically controlled unit injector is still actuated mechanically; the amount of fuel fed into the plunger-type injector pump is under the control of the engine control unit (in locomotives, locomotive control unit), rather than the traditional
Woodward PGE governor, or equivalent engine governor, as with conventional unit injectors. Specific to GM two-stroke (
6-71) and related on-road/off-road/marine two-stroke engines: • The same basic considerations are employed (the GM/EMD 567 and the GM/Detroit Diesel 6-71 engines were designed and developed at the same time, and by the same team of engineers and engineering managers). • Whereas all EMD and Detroit Diesel two-stroke engines employ forced induction, only some EMD engines employ a turbo-compressor system. Some Detroit Diesel engines employ a conventional turbocharger, in some cases with intercooling, followed by the usual Roots blower, as a turbo-compressor system would be too costly for certain very cost-sensitive and highly competitive applications.
Fuels Fuels used in diesel engines can be composed of heavier hydrocarbon oils than the petrol or
gasoline used in spark-ignition engines, making them less volatile with a higher
flash point and giving them higher
energy density. They are therefore easier and safer to handle and occupy less volume for a given amount of energy. Two stroke diesels usually burn even heavier grades of fuel oil than standard
diesel fuels. In two-stroke marine diesel engines for sea-going craft, the most common fuels are
residue oils. Günter Mau argues that no uniform standards for such fuels exist, which is why they have several different colloquial names, including
Marine Intermediate Fuel,
Heavy Fuel Oil,
Marine Bunker Fuel, and
Bunker C Fuel. Heavy fuel oils were also used in the Jumo 205 two-stroke diesel aircraft engine. In the 1960s, residue oils were "concocted on the basis of refinery waste". Residue oils are of very low quality with high viscosity and low
cetane numbers, but cheap and thus economical to use. ==Manufacturers==