Direct Injection vs Port Injection

Fuel injection has come a long way, with different systems designed to boost engine performance and efficiency. Two of the most common types are port fuel injection (PFI) and direct fuel injection (DFI). While both get fuel into the engine, they do it in different ways, each with its own pros and cons. Whether you're a car enthusiast, an engineer, or just someone curious about how your vehicle works, it's helpful to know the differences between these systems.

Although many modern engines, like Ford's Coyote platform, combine port and direct fuel injection to produce the most amount of performance possible, this is a recent development. In this article, we'll break down what sets port and direct fuel injection apart and how each one affects things like fuel efficiency, power, and emissions.

What is Port Injection?

Port injection is a form of fuel injection where fuel is injected prior to the actual cylinder where combustion happens. As the name would suggest, port injection occurs in the intake port. Individual fuel injectors squirt fuel into the different channels of the intake manifold, upstream of each cylinder's intake valve, and this fuel is then mixed with air before traveling into the cylinder for ignition.

After the air and fuel have been combined, the intake valve opens and pulls this mixture into the cylinder for ignition. This ignition pushes the cylinder down and spins the crankshaft.

One of the biggest distinctions between port injection and direct injection is the time at which the fuel is being sprayed. In port injection engines, fuel enters the cylinder during the intake stroke, but the fuel can be sprayed during any of the four engine strokes. Port injection also only require a single fuel pump.

Advantages of Port Injection

Port injection has a few important advantages over direct injection. One of the biggest reasons its so popular is that it's simple and inexpensive to produce engines with port injection. Because port injection requires less moving parts, port injection engines are less complex and require less development time and materials. Port injectors are also less complex and cheaper.

Another advantage to port injection is fuel vaporization. Because the fuel is injected before the valve and cylinder, it gets time to fully vaporize before entering the combustion chamber. This allows for a more reliable burn. The fuel also cleans the valves in port injection engines, leading to less carbon build-up over time.

The final and most important reason port injection is so popular is the ability to run effectively at high RPMs. Because fuel can be sprayed at any time during the combustion cycle, you don't have to worry about fuel starvation. This leads to port injection engines having higher RPM redlines than strictly direct injection engines.

Disadvantages of Port Injection

While port injection sounds great, there are still plenty of disadvantages that can't be overlooked. Port injection is less efficient than direct injection because not all the fuel that gets sprayed enters the combustion chamber. This can lead to wasted fuel and buildup in the intake manifold.

This wasted fuel also leads to increased emissions and reduced fuel economy. However, port injection is actually more efficient than direct injection at low speeds. But, direct injection is more efficient across the whole RPM range.

The final disadvantage to port injection is lower engine compression ratios. Because fuel is in the chamber for longer, it becomes much hotter than fuel in a direct injection engine. Hot fuel is more likely to spontaneously combust. This causes engine knock and potentially ruin port injection engines. To prevent this, port injection engines have more conservative compression ratios. This leads to decreased performance and overall efficiency.

What is Direct Injection?

Direct injection is a form of fuel injection where fuel is sprayed directly into the cylinder. This process occurs during the compression stroke of the engine at incredibly high speeds and pressures. Direct injection engines are significantly more complex than port injection because of the increased pressure of the cylinder and the split-second timing required.

Direct injection engines use two fuel pumps to ensure that the cylinders are getting highly-pressurized fuel at all times. Also, direct injection fuel injectors are more expensive than port injectors because they're larger and more complex.

Advantages of Direct Injection

One of the most obvious advantages of direct injection when compared to port injection is the improved performance, fuel economy, overall efficiency, and reduced emissions of direct injection engines. Because all of the fuel sprayed in the cylinder is burned with zero waste, direct injection engines are incredibly efficient. This efficiency leads to improved compression ratios, and improved compression ratios lead to better performance. Direct injection is preferred for high-performance applications because of this.

Disadvantages of Direct Injection

While direct injection sounds great, there are plenty of downsides as well. One of the most commonly talked about issues with direct injection is the poor combination of air and fuel at low RPMs. Because the fuel is sprayed at such a late stage in the combustion process, the fuel doesn't have enough time to completely mix with the air from the intake. This causes low efficiency at lower engine speeds. Another disadvantage to direct injection is the increased emissions at low engine speeds. For the same reasons listed above, emissions increase at low RPMs.

However, the biggest disadvantage to direct injection is the increased maintenance cost associated with it. Replacing broken or damaged components in a direct injection engine is expensive because they're much more complex than port injection engines. This leads to increased general maintenance costs associated with the parts and labor needed to keep direct injection engines working properly.

What is Dual Injection?

Luckily for engine manufacturers, there is a solution to fuel injection where you can get the best of both worlds. Dual injection combines the best aspects of port injection and direct injection to create an efficient engine at low RPMs that doesn't run out of fuel at higher RPMs.

Dual injection engines combine both a port injector and direct injector to supply the cylinder at different times directed by the car's ECU. At lower RPMs, the ECU will use the port injector to get the most out of the engine. Then, as the engine progresses through the RPM range, the ECU will switch to the direct injector to improve performance, fuel efficiency, and more.

Many modern engines feature dual injection, including Ford's own Coyote engine. This is the ideal solution for most manufacturers, but it's an incredibly complicated system that isn't available on every new car. As the technology becomes more widespread and cheaper, more and more modern cars are sure to have dual injection.

Which Fuel Injection System Is Right for Your Car?

While there isn't a right or wrong answer for the this question, there are definitely certain applications where direct, port, or dual injection is best. If you're looking to perform an engine swap on your classic Mustang, be sure you understand the differences between the different types of fuel injection to make the right decision.

Looking to get some new injectors for your Mustang? Check out our comprehensive selection below.

Source: PORT vs DIRECT vs DUAL INJECTION - a DETAILED comparison -EVERYTHING you need to know in 1 video, driving 4 answers

Image Credit: Tech Through Time: How Fuel Injection Works, carsguide | Gasoline direct injection, Bosch | Ford Mustang GT's Gen 4 Coyote V8 Now Available As A Crate Engine, CarBuzz | Gasoline port fuel injection, Bosch | DBV2 | How it Works - PFI vs GDI Engines, Lubrizol360