An Introduction Fuel Systems in Internal Combustion Engines

Hello again fellows, in this post I will attempt to provide an overview of the most widely used techniques for fuel management systems in internal combustion engines. That said, you should have some very basic knowledge of this type of engines to understand this blog post. You can read my brief introduction here. Although, my posts focus mainly on gasoline engines, the below information applies to diesel ones too. So, here we are…

1) Carburetor (aka carb)
This is the “old school” type for getting the air/fuel mixture into the combustion chambers. Although it is very rarely used in modern vehicles it was the most common type used until the early 90s in both cars and bikes. Its design is straightforward as you can see in the figure below.



Now, looking from right to left in the above figure we have the following two paths for each of the mixture’s elements…
The fuel goes like this:
– The fuel pump inside the gas tank pumps gas when the starter begins
– The fuel line drives the fuel to the fuel filter
– At last, gas enters the float bowl of the carb
On the other hand, the air has an even simpler path, which is:
– If carb has air filter/cleaner, it passes through it
– Air enters the carb’s air intake
Before moving on, here is a real carb kit for V-Twin engines designed by S&S that will hopefully help you apply the above diagram information to a more realistic case.



In this picture, apart from the carb you can also see the air filter/cleaner as well as its cover, a small pipe that is used to distribute the output mixture of carb to the two cylinders of the V-Twin engine (known as intake manifold) and all the gaskets, clamps, extra fuel lines and screws required to mount this kit.

Now that you hopefully have a better picture of the whole thing, let’s jump back to the carb’s operation. In the figure I gave it is not very clear what happens to the fuel after it enters the float bowl. What it happens is that it moves to an adjustable tiny hole that let’s it flow inside the main body of the carb. Now, you can see a butterfly valve named throttle plate. This is commonly connected through a line to the accelerator pedal or lever depending on the vehicle. The more you push the accelerator, the more that valve opens delivering more air/fuel mixture to the engine. Since the delivered mixture should not be the same for all engines, carb’s provide some simple screws (look at the given picture for speed and mixture screws) that can be used to adjust the fuel jet and butterfly valve to provide the best possible mixture.

I’m fairly sure that by know you should be wondering what’s with that “choke plate” at the very entrance of air in the carb’s body. Before explaining this you should be aware that gasoline (liquid form) is not exactly flammable, its vapor is. In order to have ignition you must have gasoline that vaporizes in a good rate. You can do a simple experiment to see this working, take a small bowl filled gasoline and place a lighted match close to its vaporizing surface, it will fire up the vaporizing gas. However, if you place it fast inside the bowl, it will quench the fire of the match. That said, you can easily understand that the colder the gasoline, the lower its vaporization rate will be. For that matter, when you start your vehicle with a cold engine is called a cold start since it creates exactly this problem that requires rich mixture to overcome.

Such fuel systems (carb) have a great disadvantage when it comes to cold engine starting. Because of the high pressure that the incoming air creates, the fuel jet(s) cannot provide the appropriate amount of fuel and cylinders starve because of the lean mixture. To fix this, some manufacturers installed additional fuel jets but the most common case is the installation of a choke plate that limits the air flow and consequently lowers the pressure allowing a smooth cold start with more fuel than air (aka rich mixture). In later models, this action was performed automatically using thermal sensors that were triggering the choke plate accordingly.
Here is a picture of a carb with the air cleaner removed on a 1968 Chevy Camaro SS:



And another one from a 2008 West Coast Choppers custom V-Twin bike with an air horn intake (no air filter/cleaner):



The main advantage of this type of fuel system is its simplicity, great torque performance and averagely easy maintenance. On the other hand, it requires small adjustments on a frequent basis, it has numerous reliability issues (it cannot perform smoothly in the whole RPM range), it has very poor fuel management efficiency and high output emission rates. However, because of its torque performance it is still used in some models either by design or as a performance tuning.

2) Mechanical Fuel Injection
This was never truly popular compared to carburetors. It was starting becoming popular in the mid-50s mainly by Bosch. Below you can see a diagram of an Audi mechanical fuel injection system.



Basically, what we can see here is a completely different design concept. The whole system was pressure driven. The sensor plate was either providing more mixture or less depending on its incoming pressure. The air/fuel mixture was being injected directly into the combustion chambers through a series of fuel injectors, one for each cylinder.
The advantage of this system was its reliable operation in the whole RPM range and its efficient fuel management. However, its complexity made it hard to repair and maintain.

3) Central Port Injection (CPI)
This is a system which is not very popular in Europe since it was designed by General Motors and it is used mostly in GMC trucks. Here is a figure that shows the core component of CPI.



What you can see above is a system that provides fuel to an injector through a tube. The latter injector is placed directly into the intake manifold of the engine. The system is has many similarities in its operation to the mechanical fuel injection but it is electronically controlled by the vehicle’s Electronic Control Unit (ECU) to provide the best possible fuel in the whole RPM range. Later on, a multi-port system was designed by GM which is known as Central Multi-Point Fuel Injection (CMFI), in this case the injectors were spraying fuel in each intake port. The main disadvantage of CMFI was that it was spraying in all ports simultaneously. That resulted in very inefficient fuel management considering the 4-stroke engine operation. To fix this, GM later introduced Central Sequential Fuel Injection (CSFI) which, as its name implies, injects fuel sequentially in each intake port.
The advantage of this system is its smooth operation and averagely simple maintenance and repair. Nevertheless, all of these systems have major fuel leaking and fuel pressure issues. Finally, apart from the newer CSFI ones, their fuel consumption is very high compared to other fuel system technologies.

4) Continuous Fuel Injection System
Commonly known as Continuous Injection Systems (CIS), such systems were very popular in the late 80s in most major German firms as K-Jetronic. Here is a nice diagram to see the operation of this technology:



The main difference from the other mechanical fuel injection systems was its constant (continuous) fuel injection. The fuel distributor manages the fuel that will be used in each injector. As you can see in the above figure, the whole system’s operation is based on the pressure which is controlled with the control pressure regulator (CPR) or as in this diagram, primary pressure regulator. The warm-up regulator (WUR) is responsible for minor pressure tweaks in the intake manifold of the engine. At last, to defeat cold start problems, this type of injection includes an additional start fuel injector that provides more fuel in case of cold start.
Although this is a good design, its complexity made it quite unpopular. However, it is still used in most piston type aircraft engines for fuel injection.

5) Single-Point Electronic Fuel Injection
The simplest form of Electronic Fuel Injection (EFI), similar to the CPI (which is a single-point injection system) it looks like this:


The design is the same as CPI meaning that the fuel is sprayed through a fuel injector to the intake manifold which then mixes it with air and drives it to each intake port of the engine. This is a popular setup in many modern vehicles.

6) Multi-Point Fuel Injection
Similar to the previous one with the difference of having multiple fuel injectors.



In most cases, these injectors are placed in each intake port separately but some designs such as Toyota D4 simply use more than one fuel injectors in the intake manifold.

7) Direct Fuel Injection
This is the most common used in modern vehicles.



As you can see in the above gasoline direct fuel injection engine, apart from the intake/exhaust valves and spark plug there is a fuel injector inside the cylinder’s head. This is the direct fuel injection technology which currently is one of the most efficient designs in both performance and fuel consumption. The fuel is driven to the injectors through electronically controlled (by vehicle’s ECU) rail and each fuel injector is electronically controlled to spray fuel at a defined pressure for a specified time. This way it can provide best results in the whole RPM range and solve problems such as cold start.
A similar technology is also available in diesel engines which uses a variant known as common rail. In both cases, the setup looks like this:


Where the fuel pump drives the gas into the fuel rail and ECU controls each fuel injector’s operation electronically.

Now that I have finished with this introduction to fuel systems, here are a couple of useful additional information…

Fuel Injectors
In a cutaway, electronic fuel injectors look like this:


There are various different types of fuel injectors which are categorized based on the maximum amount of fuel they can deliver (aka their capacity).

Manufacturer Specific Names
Many manufacturers use different names for their implementation of fuel injection, below is a quick list of the most common ones.
– Mazda Direct Injection Spark Ignition (DISI)
Which is a direct injection system as its name implies.
– Volkswagen Group Fuel Stratified Injection (FSI)
Another direct injection system with some advanced ECU fuel management to provide the best amount of air/fuel mixture depending on the driving style (economic, sport, etc.) dynamically.
– Toyota D4 and D4-S
The D4 uses the intake port central injection with two separate fuel injectors and the D4-S is the direct fuel injection implementation of Toyota.
– Mercedes-Benz Charged Gasoline Injection (CGI)
This implementation focuses mainly on fuel economy using direct fuel injection.
– Alfa Romeo Jet Thrust Stoichiometric (JTS)
Another direct fuel injection system.
– Renault Injection Direct Essence (IDE)
This is also direct fuel injection but Renault’s design focuses on fuel economy instead of performance.
– General Motors Throttle Body injection (TBI)
This is a single-point electronic fuel injection system.
– Mitsubishi Gasoline Direct Injection (GDI)
From its name is fairly obvious, direct fuel injection.
– Ford Smart Charge Injection (SCi)
Which is also direct fuel injection system.
– PSA Peugeot Citroën Gasoline Direct Injection (GDi)
Using the design of Mitsubishi GDI system.

I might have missed some here but these are the most common I can recall. There are a few similar ones for turbocharged vehicles too such as:
– Volkswagen Group Turbocharged Direct Injection (TDI)
Which is the equivalent implementation for engines equiped with turbochargers.
– Ford Gasoline Turbocharged Direct Injection (GTDI)
Similar to the VW’s TDI one.

I’m fairly sure I have forgotten a few. Please if you are aware of any other leave a comment to include it in this list. :)

Fuel Injection Performance Tuning
Since power equals air/fuel mixture we can easily see that this is a crucial part of engine’s operation. However, it’s not very common seeing performance tuning especially in the fuel injection system. It usually consists of more complex tuning such as superchargers and turbochargers which always either require additional fuel injectors or simply higher capacity ones.
Nevertheless, an extremely simple and usually easy to perform tuning is re-program (in fact re-map) the map of the air/fuel ratio in vehicle’s ECU. Here is an screen capture of such a map.



In most nowadays vehicles ECU uses flash memory which makes this task extremely straightforward. There are even open source applications like ECU Remapping. However, this becomes more complex when it comes to ECUs using EPROM.

Like I said above, after some engine tuning it is common to upgrade the fuel injection or carburetor system to increase performance. For example, have a look at the below kit.



which is a pretty hardcore MoTeC Electronic Fuel Injection upgrade for the whole fuel injection system. It includes everything from high capacity fuel injectors, re-programmable ECU, the required harness, ignition system distributor, etc. Even though this is a massive upgrade it usually requires some performance tuning for more incoming air such as supercharger, turbocharger, nitrous, etc.
Similar to the MoTeC one, here is a kit for carburetor based engines:



This time by ACCEL.

So, I might have missed something here but my intention was to provide a brief introduction. In any case, if you thing I missed something let me know.

All of the pictures were taken from Google Image search, none is mine.