With descriptions of the engine control module (Part 1) and sensors (Part 2) now out of the
way, it’s time to take a look at some of the other hardware that keeps engines running. Some of these devices will be
quite familiar, and others may not seem to be immediately related to engine management or emission controls. Rest
assured, however, that all are necessary to keep things running smoothly, reliably, and cleanly.
Here’s a shot of the twin 48mm throttle blades that are used on this vehicle to regulate airflow into the engine by
deliberately inducing a controlled restriction. At low throttle openings, a partial vacuum is formed in the intake
manifold, which results in reduced cylinder filling and less engine output.
A significant drawback of a cable operated system such as the one shown here is that the throttle opening is always
proportional to the accelerator position, but the relationship between throttle opening and engine output is by no means
constant over the wide range of engine operating speeds. As a result, less throttle opening is required to produce
full power at low engine speeds than it is at higher revs, which results in more throttle response down low than up
top – especially for engines biased towards producing lower-end power. Electronic throttle controls can be
“mapped” in such as way as to directly relate accelerator position to power output, which dramatically
improves the consistency of throttle response across most operating conditions.
Circled in red is the idle air control (IAC) valve. In this case, a pintle is moved by a two-phase stepper motor,
allowing precise metering of airflow when the throttle blades are closed. It is via the IAC that idle speed is
controlled quite carefully, which improves idle emissions and decreases fuel usage. Modern engines often use changes in
ignition advance to aid in controlling idle speed, as an engine with a substantial amount of intake manifold plenum
volume may not react quickly to small changes in airflow.
The evaporative recovery system (EVAP) uses a carbon canister (not pictured due to its location under the battery)
to recover fuel vapors vented from the fuel tank as the temperature increases or atmospheric pressure decreases. The
carbon canister is then vented into the intake manifold via the EVAP valve (circle above in green). It is preferred
that this occurs during wide open throttle (WOT), as the small amount of additional fuel can dramatically affect the
air/fuel ratio at the smaller rates of airflow during part-throttle operation.
Above, we’re pointing to both ends of the positive crankcase ventilation (PCV) system. This uses engine vacuum to
ventilate (“scavenge” in gearhead terms) any fumes that accumulates in the crankcase due to blowby (a
condition by which the combustion gases leak past the piston rings). As this system uses manifold vacuum, scavenging is
lost at WOT. In the case of forced-induction vehicles with poor ring seal, the crankcase can even become pressurized,
with significant oil consumption occurring as a result. Serious competition engines regularly use external pumps to
pull vacuum at full throttle, which often results in substantially increased power due to improved ring seal and
decreased oil consumption (oil in the combustion chamber causes a severe decrease in octane and therefore an
increased propensity for detonation).
Inserted in the intake manifold are the fuel injectors; here we see two of the eight Multec-type injectors used on
this engine. The amount of fuel injected is determined by the injectors’ flow rate, the fuel pressure applied to the
injectors, and the length of time (“pulsewidth”) that the injector is open. The flow rate is essentially
fixed by the injector design, so the variable used by the ECM to control the rate of fuel flow is the injector open time
(we’ll touch on fuel pressure in a moment).
Modern engines fire each injector individually (referred to as “sequential injection”) at low engine
speeds; the goal is typically to inject fuel at a closed intake valve, as superior atomization occurs when the valve
begins to open and high-velocity air is drawn into the cylinder. At higher engines speeds and larger throttle openings,
it is no longer possible to synchronize the injector timing with the valve events, as the injector is open the
majority of the time– so the ECM switches to a “batch mode” of injection, where all injectors are opened and
closed simultaneously.
At the left rear of the LT1 is a trio of engine management devices. Circled in red is the fuel pressure regulator
(FPR). The electric in-tank pump does a poor job of regulating fuel pressure, so the FPR uses manifold vacuum to
reference the fuel rail pressure to the absolute pressure of the intake port. Many modern engines do away with the FPR
to reduce cost and eliminate a source of potential leaks in the fuel system (evaporative emissions
requirements are extremely strict nowadays), thus actual fuel pressure must be measured by the ECM and the injector
pulsewidth varied accordingly.
The exhaust gas recirculation (EGR) valve (circled in blue) and its vacuum control solenoid (in green) serve to
bring exhaust gases into the intake manifold at part-throttle and low engine speeds. The effect of EGR is to buffer and
cool the combustion process, which results in decreased oxides of nitrogen emissions. This vehicle was one of the last
with vacuum-operated EGR valves; later vehicle use a stepper motor and position sensor to more accurately
control the amount of EGR. Malfunction of the EGR valve is rather common due to the extreme
operating environment (erosion of the pintle and seat or blockage of the valve with carbon deposits
are two such failure modes), with a decrease in idle quality and part-throttle drivability often the
result. Vehicles with large camshafts (such as the LT4 version of this engine) or variable valve timing can
make use of overlap between the closing of the exhaust valve and the opening of the intake valve to provide some
“natural” EGR, and thus don’t require an EGR valve.
While electric fans aren’t normally considered to be part of the engine management system, they’re invaluable in
maintaining the operating temperature of the engine (which is absolutely critical to efficient operation) while
minimizing parasitic loss. The substantial airflow through this radiator at highway speeds allows the PCM to turn off
the fans completely above 55 MPH; at lower speeds, one fan normally takes care of cooling needs. Increased operating
temps or use of the air conditioning will cause the second fan to activate.
Part 2 of this series discussed the position sensing aspect of the Optispark distributor, but did not mention the
function of this part that actually gives it its name. High voltage is generated in the ignition coil via the flyback
principle, with a switched 12V input being boosted to over 50kV. This is sent to the distributor, where it is fed to
the rotor. As the name suggests, this part is spun by the camshaft, and delivers the high-voltage spark energy to the
appropriate wire. It’s a crude system, and one that has thankfully been replaced by individual coils (one per cylinder
or pairs of cylinders).
Catalytic converters (“cats” or “cat-cons” for short) provide a reaction site for the
oxidization or reduction of potential pollutants. The name comes from the presence of catalysts, which are usually
precious metals such as platinum deposited onto a honeycomb substrate. This allows for a large catalyst surface area
that also provides minimal airflow restriction. As such, modern cats amount to virtually no loss of
performance.
Excessively lean or hot combustion conditions form oxides of nitrogen (caused by excess oxygen combining with
naturally abundant nitrogen at high temperatures); rich or cool conditions cause hydrocarbons (raw fuel) or carbon
monoxide as a result of incomplete combustion. The dithering of the air-fuel ratio from rich to lean that we discussed
in the last post helps the cats do their job, as the momentarily rich condition reduces the oxides of nitrogen, while
the lean phase oxidizes carbon monoxide and hydrocarbons to carbon dioxide. This process requires high temperatures, so
modern engines mount the cats as close as possible to the exhaust manifold in order to minimize the amount of time
required to bring the cats up to operating temps.
Not shown here is the electric air pump used on the LT1 to pump additional oxygen to the
cats during start-up, as no dithering of the air-fuel ratio occurs until the vehicle is warmed-up and
operating under closed-loop conditions. The air pump has been eliminated on most modern vehicles by better controlling
the spark and fuel during start-up.
We hope that these posts on engine management and emission controls have been helpful to our readers, and invite
feedback on this series in the Comments section. If there is something that you would like to see investigated in more
detail, or if there are other vehicle subsystems that are of interest, please feel free to speak up and let us know.