Solar Energy International Biodiesel Workshop

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What is a Diesel Engine?


• Rudolf Diesel developed the
idea for the diesel engine and
obtained the German patent for
it in 1892.
• His goal was to create an
engine with high efficiency.
• Gasoline engines had been
invented in 1876 and, especially
at that time, were not very
efficient
• Both the gasoline and diesel
engine utilize the process of
internal combustion for power



Compression Ratio
• Compression ratio:
This is defined as the ratio of the volume of the cylinder at the beginning of the
compression stroke (when the piston is at BDC) to the volume of the cylinder at
the end of the compression stroke (when the piston is at TDC).
• The higher the compression ratio, the higher the air temperature in the
cylinder at the end of the compression stroke.
• Higher compression ratios, to a point, lead to higher thermal efficiencies and
better fuel economies.
• Diesel engines need high compression ratios to generate the high
temperatures required for fuel auto ignition.
• In contrast, gasoline engines use lower compression ratios in order to avoid fuel
auto ignition, which manifests itself as engine knock or pinging sound.
• Common spark ignition compression ratio: 8:1 to 12:1
• Common compression ignition ration: 14:1 to 25:1



Direct Injection
• Direct-Injection (DI) or Open Chamber Engine:
In this design, the fuel is injected directly into the
cylinder chamber.
Direct injection engines have two design
philosophies:
-High-swirl design, which have a deep bowl in
the piston, a low number of holes in the injector
and moderate injection pressures.
-Low-swirl or quiescent engines are
characterized by having a shallow bowl in the
piston, a large number of holes in the injector
and higher injection pressures.
• Smaller engines tend to be of the high-swirl type,
while bigger engines tend to be of the quiescent
type
• All newer diesel engines use direct fuel injection
• Much higher fuel pressure then indirect fuel
injection (example TDI )
• Injection/Injector Timing is critical
• Equipped with in-line pumps, distributor pumps,
rail injection systems, or pump injector units



Indirect-Injection Engine (IDI):
In this design, the fuel is injected into a small pre-chamber attached to the main cylinder chamber.
The combination of rapidly swirling air in the prechamber and the jet-like expansion of
combustion gases from the prechamber into the cylinder enhances the mixing and combustion
of the fuel and air.
Starting is aided by a high compression ratio (24-27) and a glow plug mounted in the pre-chamber.
This design has the advantage of less noise and faster combustion,
but typically suffers from poorer fuel economy.


Diesel Properties: Cetane
• One of the most important properties of a diesel fuel is its readiness to
auto-ignite at the temperatures and pressures present in the cylinder
when the fuel is injected.
• The cetane number is the standard measure of this property.
• Cetane – (ASTM D613) is tested by adjusting the fuel/air ratio and the
compression ratio in a single cylinder, indirect injection diesel engine to
produce a standard ignition delay (the period between the start of fuel
injection and the start of combustion).
• ASTM D6751 Biodiesel spec. has a minimum cetane number of 47
• Cetane improvers are fuel additives that are designed to readily
decompose to give precursors to combustion and thus enhance the
rate at which auto-ignition occurs.
• Typical compounds used are alkyl nitrates, ether nitrates, dinitrates of
polyethylene glycols, and certain peroxides. Due to low cost and ease
of handling, alkyl nitrates are the most widely used cetane improvers