Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
4.3.7 Heat release data for compression-ignition engines
In Sec. 4.1.6 it was pointed out that combustion by compression ignition requires the
rapid heating of the droplets in the fuel spray, initially to vaporize the liquid, and then to
promote the rise of that vapor temperature to the auto-ignition temperature in hotter air. The
smaller the droplet, but more importantly the higher the relative velocity between fuel droplet
and air, the more rapid will be the transfer of heat from air to fuel to accomplish this effect.
There are two ways this can be accomplished, either fast-moving fuel and slow-moving air, or
fast-moving air and slow-moving fuel.
The direct injection (DI) engine uses the fast-moving fuel approach by employing highpressure
injection into a bowl formed in the piston crown (see Fig. 4.1(b)) to give the highest
speed for the fuel droplets. Recent design trends are for ever-higher injection line pressures,
up to 1000 bar or more, to give more rapid motion to the fuel droplets. As the air and fuel
relative velocities are virtually fixed at any engine speed, this tends to give a low-speed limit
for the engine, typically 3000 rpm in automotive applications; there are exceptions to this
statement.
The indirect injection (IDI) engine uses the fast-moving air approach by employing compression-created
rapid swirl in a side combustion chamber into which low-pressure fuel injection
can be employed to provide only a moderate speed of movement for the fuel droplets.
The fuel injection line pressures are rarely higher than 250 bar, which also implies both a
cheaper and a quieter injection system well suited to automobile applications. As the swirling
air flow (see Fig. 4.9) is compression created, the relative velocity of air to fuel tends to rise
with engine speed and so the speed limit for this engine is higher than the DI engine. The
limiting speed tends to be that of the (mechanical/hydraulic) fuel injection equipment rather
than the combustion process.
4.3.7.1 The direct injection (DI) diesel engine
A sketch of a typical combustion chamber for such an engine is shown in Fig. 4.1(b). This
shape is often referred to as a "Mexican hat," for obvious reasons. In practice, many other DI
designs are employed from the simple "bowl in piston" as shown in Fig. 4.18 or Fig. 3.41, to
"squish lip" designs, "wall wetting," etc. [4.32].
Combustion in DI diesel engines is characterized by rapid burning around the tdc position
and a sketch of a typical heat release rate curve is shown in Fig. 4.8(a). Apart from the high
compression ratio, which in itself provides high thermal efficiency as seen in Eq. 1.5.22, the
rapid burn around tdc approaches the ideal for combustion which is a constant volume process.
However, the rapid rates of pressure rise make it extremely noisy, i.e., the typical DI
diesel "rattle."
In Fig. 4.8(a) the sharp spike of heat release close to the tdc position is known as "premixed"
combustion and the remainder as "diffusion" burning. The position of the peak of premixed
burning and the profile of the diffusion burning period tends to be influenced by engine
speed, as forecast in Sec. 4.1.6. For automotive engines, or so-called "high-speed" diesel
engines used in automotive applications, some simple information can be dispensed on these
factors for engine speeds between 1000 and 2600 rpm. Above or below these speeds, the
maximum or minimum values of 2600 or 1000 should be used in the theory below.
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