Modern SI engines mix air and fuel in the intake manifold by way of one or more
low-pressure (50-psi or so) injectors. A throttle valve regulates the amount of air
admitted, which is only slightly in excess of the air needed for combustion. As the
throttle opens, the injectors remain open longer to increase fuel delivery. For a gasoline
engine, the optimum mixture is roughly 15 parts air to 1 part fuel. The air-fuel
mixture then passes into the cylinder for compression and ignition.
In a CI engine, air undergoes compression before fuel is admitted. Injectors
open late during the compression stroke as the piston approaches tdc. Compressing
air, rather than a mix of air and fuel, improves the thermal efficiency of diesel
engines. To understand why would require a course in thermodynamics; suffice to
say that air contains more latent heat than does a mixture of air and vaporized fuel.
Forcing fuel into a column of highly compressed air requires high injection pressures.
These pressures range from about 6000 psi for utility engines to as much as
30,000 psi for state-of-the-art examples.
CI engines dispense with the throttle plate—the same amount of air enters the
cylinders at all engine speeds. Typically, idle-speed air consumption averages about
100 lb of air per pound of fuel; at high speed or under heavy load, the additional
fuel supplied drops the ratio to about 20:1.
Without a throttle plate, diesels breathe easily at low speeds, which explains
why truck drivers can idle their rigs for long periods without consuming appreciable
fuel. (An SI engine requires a fuel-rich mixture at idle to generate power to overcome
the throttle restriction.)
Since diesel air flow remains constant, the power output depends upon the
amount of fuel delivered. As power requirements increase, the injectors deliver more
fuel than can be burned with available oxygen. The exhaust turns black with partially
oxidized fuel. How much smoke can be tolerated depends upon the regulatory
climate, but the smoke limit always puts a ceiling on power output.
To get around this restriction, many diesels incorporate an air pump in the form
of an exhaust-driven turbocharger or a mechanical supercharger. Forced induction
can double power outputs without violating the smoke limit. And, as far as turbochargers
are concerned, the supercharge effect is free. That is, the energy that drives
the turbo would otherwise be wasted out the exhaust pipe as heat and exhaust-gas
velocity.
The absence of an air restriction and an ignition system that operates as a function
of engine architecture can wrest control of the engine from the operator. All
that’s needed is for significant amounts of crankcase oil to find its way into the combustion
chambers. Oil might be drawn into the chambers past worn piston rings or
from a failed turbocharger seal. Some industrial engines have an air trip on the intake
manifold for this contingency, but many do not. A runaway engine generally accelerates
itself to perdition because few operators have the presence of mind to engage
the air trip or stuff a rag into the intake
low-pressure (50-psi or so) injectors. A throttle valve regulates the amount of air
admitted, which is only slightly in excess of the air needed for combustion. As the
throttle opens, the injectors remain open longer to increase fuel delivery. For a gasoline
engine, the optimum mixture is roughly 15 parts air to 1 part fuel. The air-fuel
mixture then passes into the cylinder for compression and ignition.
In a CI engine, air undergoes compression before fuel is admitted. Injectors
open late during the compression stroke as the piston approaches tdc. Compressing
air, rather than a mix of air and fuel, improves the thermal efficiency of diesel
engines. To understand why would require a course in thermodynamics; suffice to
say that air contains more latent heat than does a mixture of air and vaporized fuel.
Forcing fuel into a column of highly compressed air requires high injection pressures.
These pressures range from about 6000 psi for utility engines to as much as
30,000 psi for state-of-the-art examples.
CI engines dispense with the throttle plate—the same amount of air enters the
cylinders at all engine speeds. Typically, idle-speed air consumption averages about
100 lb of air per pound of fuel; at high speed or under heavy load, the additional
fuel supplied drops the ratio to about 20:1.
Without a throttle plate, diesels breathe easily at low speeds, which explains
why truck drivers can idle their rigs for long periods without consuming appreciable
fuel. (An SI engine requires a fuel-rich mixture at idle to generate power to overcome
the throttle restriction.)
Since diesel air flow remains constant, the power output depends upon the
amount of fuel delivered. As power requirements increase, the injectors deliver more
fuel than can be burned with available oxygen. The exhaust turns black with partially
oxidized fuel. How much smoke can be tolerated depends upon the regulatory
climate, but the smoke limit always puts a ceiling on power output.
To get around this restriction, many diesels incorporate an air pump in the form
of an exhaust-driven turbocharger or a mechanical supercharger. Forced induction
can double power outputs without violating the smoke limit. And, as far as turbochargers
are concerned, the supercharge effect is free. That is, the energy that drives
the turbo would otherwise be wasted out the exhaust pipe as heat and exhaust-gas
velocity.
The absence of an air restriction and an ignition system that operates as a function
of engine architecture can wrest control of the engine from the operator. All
that’s needed is for significant amounts of crankcase oil to find its way into the combustion
chambers. Oil might be drawn into the chambers past worn piston rings or
from a failed turbocharger seal. Some industrial engines have an air trip on the intake
manifold for this contingency, but many do not. A runaway engine generally accelerates
itself to perdition because few operators have the presence of mind to engage
the air trip or stuff a rag into the intake
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