Direct Injection vs Port Injection

The automotive world is constantly coming up with improvements to increase efficiency and performance. Along these lines, one of the best pieces of technology is gasoline direct injection, or GDI for short. GDI is a type of electronic fuel injection (EFI) system that is more efficient, produces better performance, reduces emissions, and improves fuel economy, compared with port injection. Direct injection vs port injection is one of the biggest topics in the car and truck world today.

This article will cover everything you need to know about DI and EFI. First, we’ll explain the history of electronic fuel injection and explain why GDI is such a big innovation. Then, we’ll go over exactly how GDI works and how direct injection vs port injection differs, including both the benefits and drawbacks of each. Finally, we’ll talk about the best engines today that use direct injection to show you how effective it is.

The History of Electronic Fuel Injection in Automobiles

Since automobiles started to become commercially produced in the early-1900s, they have not widely used electronic fuel injection (EFI). With some exceptions, until the late-1980s, auto manufacturers exclusively relied on carburetors. Carburetors perform the task of mixing fuel and air together via the venturi effect, and were the standard for decades.

That’s not to say there were absolutely no fuel injection systems prior to the ‘80s. General Motors invented the Rochester Ramjet in the 1950s, which was a mechanical fuel injection system. GM used the Ramjet on many passenger cars, most famously in the Chevrolet Corvette from 1957–1965. There was also the Bosch injection system from Mercedes in the 1950s, as well as various injection systems used in aircraft engines. There was also the Bendix Electrojector system, which was an early but very unreliable EFI system.

By the 1960s, fuel injection was becoming more common in racing applications, but was not really used for production vehicles. This was mainly due to its unreliability and costs. However, in the late-1960s, using technology developed from the Electrojector, Bosch came out first with the D-Jetronic and then K-Jetronic fuel injection systems.

The Ramjet and Jetronic systems were both mechanical fuel injection systems, and the Electrojector was a primitive electronic fuel injection system. The first widespread electronic fuel injection (EFI) systems were put into use in the 1980s. By the end of the decade, pretty much every major manufacturer, like Ford and GM, were beginning to make EFI systems standard in all of their automobiles.

Introducing Direct Injection

Since 1994, every car and truck produced in and for the U.S. market uses some form of EFI instead of carburetors. The last holdouts were the Subaru Justy, Honda Prelude, and Isuzu pickup, as well as a few Fords and Chevys that lasted until 1991–1992. In comparison with carburetors, EFI systems conserve fuel and improve fuel economy, improve performance, and reduce tailpipe emissions – an increasing environmental concern as the true cost of engine emissions began to be discovered.

There have been many improvements on EFI systems throughout the years, making them more efficient, reliable, and cost effective. Since the 1990s, multiport and sequential EFI systems have been pretty much standard in all cars and trucks. By the early 2000s, most kinks had been worked out, and EFI systems are widely regarded as superior to carburetors.

In the 2010s, a new form of EFI known as gasoline direct injection (GDI or just DI for short) began to enter into widespread production. GDI is more efficient, produces better fuel economy, and reduces emissions, compared with older EFI systems – which are also referred to as port injection systems. Mitsubishi came out with the first GDI system in the 1990s, but it wasn’t until the 2010s GDI systems became more widely used.

Today, there are many manufacturers using DI systems, among them are Ford, Subaru, Mercedes-Benz, Chrysler, Mazda, and BMW. In general, DI systems are more expensive to manufacture, install, and keep running, and some have run into reliability issues. However, as the kinks are worked out, manufacturers have started to embrace direct injection systems in more and more applications. Many manufacturers also now use hybrid systems that utilize both standard EFI and DI systems in conjunction with each other to minimize the pitfalls of both.

Electronic Fuel Injection Explained

Before getting into the intricacies of direct injection, first let’s talk about what exactly electronic fuel injection (EFI) is and how the various different kinds work. As we mentioned, before there was electronic fuel injection there was mechanical fuel injection, like the GM Rochester Ramjet and Bosch Jetronics. These work by having a fuel pump driven by the crankshaft that continuously supplies fuel, and the fuel pressure rises as engine speed and load increases.

In contrast, EFI works with the Powertrain Control Module (PCM) being responsible for controlling injector flow and fuel pressure. The PCM takes measurements from various sensors, including the O2, Mass Air Flow (MAF), throttle position, crankshaft, and camshaft sensors, and determines the most optimal fuel pressure. With the PCM/ECU in charge of everything, fuel delivery is much more precisely controlled. This means better fuel economy and performance, which in turn means reduced emissions for the same power.

Compared with carburetors, EFI systems operate at much, much higher fuel pressures, which is supplied by an electric fuel pump. In addition, the injector nozzles have umbrella patterns on the top, which increases fuel atomization. The fuel injectors are usually placed in the intake manifold, and they supply fuel through the manifold ports and into the combustion chambers.

Single-point, Multi-point, and Sequential Fuel Injection

You have probably heard the terms single-point, multi-point (or multiport), and sequential fuel injection, and here’s what they mean. Single-point injection was used on the earliest mass produced EFI systems in the 1980s. In these systems, there was a single fuel injector (or sometimes two) that was placed inside the throttle body. Fuel would be injected into the intake manifold, mix with air, and travel through the runners into the combustion chamber.

Multi-point fuel injection was first brought out in the mid and late-1980s, and is still widely in use today. In multi-point systems, there are one (or sometimes two) injectors per cylinder instead of per engine. As you can imagine, multi-point systems are much more precise and allow for much better control of fuel delivery.

Sequential fuel injection is an advanced type of multi-point injection. The first EFI systems were primitive and had multiple injectors fire at the same time in groups or “batches.” Sequential fuel injection became standard in the 1990s, and allows each injector to fire independently of the others. This allows for even greater control of fuel delivery for even better performance, fuel economy, and reduced emissions.

Direct Injection Explained

Finally, we have arrived at direct injection. Direct injection is a form of sequential multi-point EFI that runs on even higher fuel pressure than port injection. Typically, standard port injection fuel pressure will run at about 25-65 PSI. In comparison, direct injection systems run at upwards of 2,000–3,500 PSI of fuel pressure.

The increased fuel pressure comes courtesy of a secondary fuel pump that is capable of extremely high fuel pressures. Known as high pressure fuel pumps, these are usually located in the engine bay, much closer to the injectors and fuel rails than the standard in tank fuel pumps.

In addition to running at higher fuel pressures, DI systems can also alter the injector’s pulse width and timing more precisely and to greater effect than port injection systems. By optimizing when fuel is injected during the combustion process and how much fuel it injects (pulse width), even more efficient fuel delivery can be achieved to boost fuel economy and performance.

Direct Injection vs Port Injection: Differences

So, besides fuel pressure, injector pulse width, and injector timing, what are the differences between direct injection vs port injection? The main difference in these systems is where the fuel injector is located. On standard port injection systems, the fuel injector sits in the intake manifold. It squirts atomized fuel into manifold ports that lead to the combustion chamber. When the intake valve opens on the combustion chamber, the fuel and air mixture make their way in.

On direct injection systems, the fuel injector sits in the cylinder head instead of intake manifold. Instead of squirting fuel into manifold ports the injector instead squirts the atomized fuel directly into the cylinder. DI fuel injectors are different from port injectors. Usually, DI injectors are piezoelectric injectors, and they are much more precise, efficient, and expensive than port injectors.

Benefits

The benefits of direct injection vs port injection are numerous. One of the biggest is increased fuel economy. By being able to more precisely control the pulse width and injection timing, fuel mapping can be greatly improved. The extreme fuel pressure means injectors don’t have to open as long, allowing for better control. In addition, the fuel getting injected straight into the cylinder allows for less fuel to be used overall. This means better fuel economy across the board.

The improved fuel economy also has another important benefit, improved emissions. GDI systems have much better tailpipe emissions than port systems. This is extremely beneficial for cold starts, which is often when emissions are at their worst. With increasing emissions restrictions due to climate change, GDI helps mitigate some of the hassles of remaining compliant.

The most important benefit, however, is the increase in performance. The increased control over fuel delivery and timing and ability to inject much greater amounts of fuel at higher loads via increased fuel pressure, means GDI systems can be tuned to make torque much quicker than non-GDI systems. Especially when paired with a twin-scroll turbo, like many are, full torque can be achieved as early as 1,500 RPM on some engines and last clear past 5,000 RPM.

Drawbacks

While direct injection is definitely nice, it’s not without its drawbacks. Most prominently, and the reason it has taken so long to be put into mass production, is cost. Everything about GDI systems, from the injectors, to fuel rails, to secondary fuel pumps, and etc., is all more expensive than port systems. This is largely because GDI systems use much more advanced technology, necessitating more expensive materials.

In addition, and this is probably the biggest drawback of DI systems, is the issue of carbon buildup. While mounting the injector in the cylinder head where it can reach directly into the combustion chamber helps with fuel mapping, it has an unfortunate downside.

On port injection systems, fuel from the injector washes over the intake valve(s) on its way into the cylinder. This has the effect of keeping the valves clean of any excess carbon residue and buildup. On GDI systems, since the fuel is injected directly into the cylinder and never touches the intake valves, the valves eventually become caked with carbon buildup.

Increased carbon buildup can result in poor performance and poor fuel economy. If it gets bad enough, it can result in misfiring and seized valves that are stuck closed. Most of the time, especially on newer GDI systems, carbon buildup is of minimal concern and typically does not show problems under 100,000 miles.

In addition, many manufacturers are using EFI systems that have both port and direct injection. These EFI systems have the best of both worlds. They have the precise fuel timing of GDI systems, but they eliminate the issue of carbon buildup due to supplemental port injectors. They are also more expensive to design and implement.

Walnut Blasting for Direct Injection

The best proven way to clean intake valves from excessive carbon buildup is known as walnut blasting. This process involves spraying walnut shells at the intake valves to remove the carbon. It’s a harmless and relatively easy, if tedious, process.

As we mentioned, most cars will not need this service until well past 100,000 miles. However, most DI cars will likely need at least one walnut blast at some point during their service history. For those feeling brave enough to try and walnut blast their own valves, we made a DIY YouTube video to explain how. Caution: This is a serious car maintenance service and should only be undertaken by qualified and knowledgeable technicians.

What Engines Use Direct Injection

Now that GDI systems have proven their efficacy, more and more manufacturers are designing GDI based engines. One of the first was BMW, who introduced direct injection in 2007 with their N54 engine. By now, all BMWs stateside are powered by direct injection, and BMW is creating some of their most powerful – and efficient – motors yet.

Ford has used Direct Injection since the 2010 model year in their EcoBoost series of engines. This includes engines like the 2.0 EcoBoost, which powers everything from sedans to SUVs and trucks. Like BMW, all EcoBoost engines are also mated with turbochargers, which play very well with DI systems.

Subaru has used DI systems in their cars since the flat-flour FA20 engine came out in 2012. The engine was a joint venture with Toyota. In the turbo variant, the FA20DIT, DI is the only fueling method. However, on the naturally aspirated version, the FA20 uses Toyota’s D-4S EFI system. The D-4S uses both DI and port injection, which mitigates problems like carbon buildup, and has appeared in many Toyota engines.

Many other manufacturers, like Mazda, Mercedes-Benz, General Motors, and Chrysler (Stellantis) all use DI systems in at least some of the engines. It is quickly rendering older port injection systems obsolete, especially in the face of increasing emissions restrictions.

Direct Injection vs Port Injection Conclusion

As you can probably tell by now, the future – if it continues to include the internal combustion engine – will likely be flooded with direct injection systems. It is already the standard among some manufacturers, and is quickly gaining a loyal following every day.

Compared with older port injection systems, DI systems are better in pretty much every sense. They allow for much more precise fuel mapping, which means less emissions, better fuel economy, and better performance. Combined with twin-scroll turbochargers, DI engines can make torque extremely early and carry it nearly to redline. Manufacturers use them in everything from high performance sedans and coupes, to ¼ ton SUVs and trucks.

The only downside for direct injection vs port injection systems is the increased costs and issue of carbon buildup. The advancement in technology and additional parts make DI systems much pricier than their port injection predecessors. In addition, as we mentioned, the carbon buildup on the intake valves is another problem, though there are ways to mitigate it.

Do you have any more questions about direct vs port injection? Let us know in the comments below!