Free Automotive Guide: Internal Combustion Engine-Fuel & Air Delivery

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CamIn this, our final installment on the internal combustion engine, we will discuss fuel and air delivery. The modern internal combustion engine requires compression, ignition spark, fuel, and air in order to operate. In this installment we will cover and air delivery along with a brief overview of how the internal combustion engine operates. If you have questions or concerns about how individual components operate, I refer you to these previous installments:

a.      Free Automotive Guide: Internal Combustion Engine – Short Block
b.      Free Automotive Guide: Internal Combustion Engine – Top End
c.       Free Automotive Guide: Internal Combustion Engine – Lubrication System

The internal combustion, or four stroke engine, is operated using — yep, you guessed it — four strokes. You could divide the strokes as such:

The intake stroke begins at top dead center and uses vacuum to draw a precise mixture of air and fuel into the appropriate cylinder at the correct time, in much the same manner as you and I draw air into our lungs in order to breathe. At or near the top of each cylinder, there is one or more intake valve/s. As the piston reaches top dead center and begins to retract, this intake valve/s is opened in reaction to the rotation of the camshaft (see segment a.). A precisely metered fuel and air mixture is delivered into the intake passage, so that when the intake valve is opened, the piston will draw it into the cylinder.

The compression stroke begins at bottom dead center and forces the air and fuel mixture into a smaller area, compressing it in the process. This compression is vital to engine operation and without a sufficient amount, combustion will not occur.

The power stroke is initialized when an ignition spark, which is created by the spark plug that is threaded into the cylinder, creates an explosion with the compressed air and fuel mixture. This explosion causes the piston to retract from the cylinder in a direction that is towards the crankshaft. As this is taking place, this cylinder’s off-setting cylinder is producing the exact opposite effect, due to the shape of the crankshaft. As a simple example a four cylinder engine with a firing order of 1-3-4-2 will be performing the intake stroke on cylinder 1, while it is performing a power stroke on cylinder 4 and cylinder 2 will be in compression stroke while cylinder 3 is performing an exhaust stroke. This keeps the engine running smoothly and in succession.

The exhaust stroke begins after the piston has reached its lowest point of the power stroke. It is then pushed back into the cylinder causing the spent exhaust gases to be emitted as the exhaust valve is opened, in the same manner as the intake valve.

This entire procedure takes place in each cylinder for every engine revolution. Engine revolutions are measured in the number per minute, or RPMs.

I trust that provides you with a little better understanding of actual combustion engine theory and operation. Now, we will turn our attention to fuel delivery and air induction.

First, let’s take a look at the determining factors involved in achieving the correct air/fuel ratio. There are a number of viable gasoline fuel alternative sources for the modern internal combustion engine; ethanol, natural gas, propane, and alcohol just to name a few. As it stands today, gasoline remains the most popular and consistent of these. That is precisely why I have chosen to deal with the gasoline powered internal combustion engine in these articles.

While the gasoline powered internal combustion engine requires gasoline in order to operate, an engine which is being delivered too much fuel will suffer from reduced performance. The same is true of air. Without air the internal combustion engine cannot operate. If you have ever placed your hand over the top of a carburetor, then you can attest to that fact. Still, excessive air will hinder the engine’s performance. Fuel and air must be delivered in precise increments (typically 15 parts air to 1 part fuel) in order for optimum engine performance to be realized.

Believe it or not, the simplest means for describing fuel delivery to a modern internal combustion engine is through a computerized engine controller, also called a powertrain control module (PCM). Intake air for the engine is confined to a single source. Obviously, the more that the throttle plate is opened the more air is introduced to the engine, hence the more fuel is required to achieve our targeted air/fuel ratio. The single source for air induction on the naturally aspirated gasoline engine (and we are going to deal exclusively with a fuel-injected version, here) is through the air inlet tube. The air inlet tube supplies the engine with clean, filtered, fresh air from outside of the car. Fresh air is pulled in using vacuum created during the intake stroke (mentioned above), through a filtration device that may or may not be fixed inside of a box. Often aftermarket (and some OEM versions, nowadays) cold-air intakes do not make use of an enclosure but pull in denser, fresher air at an increased rate using heat shields to gain access to outside air. As this filtered air is drawn through the inlet pipe, a sensor measures the volume. This sensor is known as a mass airflow (MAF) sensor. As the throttle is opened, creating increased airflow, a voltage signal from the (MAF) to the PCM is increased. The PCM utilizes this signal, along with other signals, including:

Oxygen sensors measure the amount of fuel being emitted through the engine exhaust. Voltage is increased with the presence of greater amounts of fuel and decreased with less.

The throttle position sensor delivers a voltage signal to the PCM which is compared with the MAF to determine if the two are within acceptable specifications. The PCM is calibrated to expect a certain amount of air at a particular throttle degree, if the difference between the two is excessive, a code will be set in the PCM, triggering a service engine light.

The manifold air pressure sensor uses voltage to measure vacuum in the intake. A voltage signal is delivered constantly to the PCM, which compares the pressure voltage to thousands of other readings simultaneously.

Camshaft and crankshaft timing are measured using the camshaft and crankshaft position sensors. Voltage signals from these two sensors are sent to the PCM and compared with all of the other readings to determine how much fuel to deliver to the engine and when.

These are only a few of the sensors which dictate air/fuel delivery in the OBD-II engine control system. The PCM gathers data and determines fuel injector pulse width and fuel pressure in order to deliver the precise air/fuel ratio to the proper cylinder at the correct instant.

Any ambient air which is introduced into the engine and is not metered by the mass airflow sensor, creates a performance problem. With the PCM being sent a counterfiet signal, it delivers the incorrect amount of fuel creating a lean condition. A lean condition occurs when there is excessive air, or insufficient fuel, being delivered to one or more cylinders. This can be caused by a vacuum leak, exhaust gas recirculation (EGR) valve malfunction, or low fuel pressure. Common causes of low fuel pressure include a faulty fuel filter of fuel pump. A rich condition is created by either an excessive amount of fuel, or an insufficient amount of air, being delivered to one or more cylinders. Typical causes of a rich condition are a clogged air filter or faulty MAF.

Please see our article on Top Automotive Innovations: Components of Electronic Fuel Injection for more on fuel delivery for the internal combustion engine.

S.M. Darby

S.M. Darby

I am a freelance author with over 25 years of experience as a professional, ASE certified automotive technician and shop owner, muscle car enthusiast, avid street racer, and classic car restoration specialist.