There have been several significant shifts in automotive fuel delivery over the past half-decade or so. The first shift started with manufacturers gradually adopting electronic fuel injection through the 1970s. EFI popularity spiked and completely eliminated carburetors from production vehicles by the 1990s. Much like EFI and the carburetor, the second shift has been from Multi-Port Fuel Injection (MPFI, or just port injection) to Gasoline Direct Injection (GDI, or just DI). Direct Injection systems slowly became more common in the mid-2000s, and have enjoyed a boom in popularity in the twenty-teens.
However, instead of phasing out port injection EFI, we’re actually starting to see a rise in popularity of hybrid (not the electric kind) dual-injection systems on everything from boring commuter cars, to gasoline pickup trucks, to performance vehicles – most notably the Gen-3 Coyote in the 2018-up Ford Mustang GT and the Gen-V LT5 engine powering the 2019 Corvette ZR1.
At this point, you might be asking yourself, “Self,” you say. “Why would they be putting that inefficient old port injection on the same engine that has a modern, high-tech direct-injection fuel system on it?” Luckily, we have someone like Jason Fenske of Engineering Explained to go over that question, and explain the benefits of a dual-injection setup.
The Basics of Both Types of Injection
“The fuel needs to change from a liquid into a gas – it needs to vaporize – before it can combust,” Fenske explains right off the bat. “To accomplish that, there is a little bit of a different strategy between port and direct injection.” Both methods of injection accomplish the ultimate goal of vaporization, but in vastly different ways.
“Port injection operates at a much lower pressure of the two. It’s injected into the runner at basically atmospheric pressure, unless the engine has forced induction,” says Fenske of the 40-45 psi of fuel pressure standard fuel injection runs. “There is plenty of time for that air to mix with the fuel and vaporize while the mixture travels through that intake runner, down into the cylinder on the entire intake stroke, and then the compression stroke.”
On the flip side of that, the very nature of direct-injection is that fuel spends a minimal amount of time out of the injector before being ignited. “With direct-injection, the [exact] timing [of the injection] can vary. It may inject during the intake stroke, or it may inject during the compression stroke,” Fenske says of the different strategies employed under different engine load and RPM conditions.
“Either way, you’re going to have less time for the fuel to mix than you do in port-injection. As a result, much higher injection pressures are used in DI. That gives you a finer mist of fuel, making it easier for the fuel to vaporize. A secondary reason for the elevated delivery pressures is that if you are injecting into the cylinder on the compression stroke, you have some serious pressure to overcome.”
What’s Cooler Than Being Cool
As you inject fuel, regardless of the method of injection (even in carburetion) there is a cooling effect that happens to the charge mixture. As the fuel vaporizes, it’s an endothermic process. That is to say, it cools the air around it, as it happens.
“With port-injection, you are cooling that air before it enters that cylinder. Cooling that charge makes it denser, allowing you to bring more air into the cylinder [through the same sized opening],” says Fenske. “That is especially pronounced in naturally aspirated engines. That effect allows more air in, more fuel to be utilized and ultimately more power to be made.”
With direct-port injection, you lose that pre-intake-valve cooling, but you gain another different, yet highly beneficial type of cooling within the cylinder. “Direct-injection has a unique advantage to cooling just within the cylinder itself. While the intake valve has closed, you are cooling the charge as it’s being compressed, and that greatly reduces the likelihood of knock,” explains Fenske. “By reducing the factors for knock, you can advance the timing, you can raise the boost, you can raise the compression ratio, and ultimately you can make more power.”
Best of Both Worlds
With both methods offering benefits, it seems like the only logical thing to do would be to use both of them, right? “With dual injection methods, the strategy will be altered based on where you are on the map, RPM and load-wise,” says Fenske of how the two systems are employed on one engine.
“Each manufacturer has a different strategy for this, but generally speaking, you are going to use port injection at lower loads, and lower RPM. Then in the midrange – again, depending on the manufacturer’s specific strategy – both port and DI will be used simultaneously. As you get into the higher-load and higher-RPM areas where you really want to prevent knock, it switches to purely direct injection.”
Fenske points out that while the exact crossover and implementation points on the map will vary with everyone’s different strategies, the theories are the same. “At low to medium load and RPM, your goal is to achieve better air-fuel mixtures, which is a benefit of port injection, because of all the extra time it has to vaporize and mix. That provides a more stable combustion and a more efficient combustion with the more even distribution of air and fuel.”
Fenske continues, “In the high-load, high-RPM region, you want to maximize the cooling effect within the cylinder itself to maximize power, as well as reducing the knock probability, so that you can take more power-making steps.”
If you watch the video in its entirety, Fenske goes on to use the Toyota D-4S system and strategy as an example of one of the more complex methods of employing dual injection, which we aren’t even going to try and explain ourselves – A) because Fenske understands it far better than we do, and B) No one explains things like Fenkse.
We’ve discussed the formation of carbon deposits on intake valves before, and spoken a bit about how direct injection affects that, by not having the fuel spray on the back of the intake valve, enjoyed by port injection. Fenske explains how the blow-by that is fed back into the intake tract – along with oil mist from excessive crank windage – by the Positive Crankcase Ventilation system, along with the Exhaust Gas Recirculation system can cause the dreaded carbon build up on the back of intake valves.
This has not gone unnoticed by the OEMs, though none have publicly stated as much. Fenske confirms the fact that no one will admit to it, but also furthers our suspicions that part of the reason for dual-injection setups is to regain the resistance to carbon deposits on intake valves. Fenske points out that he found a 2011 SAE study that shows ten-times the amount of deposits found on direct injected engines over port-injected engines. So while no one is admitting that as a reason for dual-injected setups, it is surely an additional benefit.