Direct Injection Piston Design and Tuning Theories

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There’s a new acronym floating around the performance industry—GDI—and it stands for gasoline direct injection. Among domestic production engines that have jumped heavily into the GDI segment are the Chevrolet’s Gen V LT-based engines, but Ford was the first to take on a spark-ignited-gasoline, direct-injected engine in 2010 with its series of EcoBoost engines.

What is GDI and why could it be the future for all production gasoline engines? It’s all about combustion efficiency. For decades, a majority of engine-building attention has focused on tuning with intake and exhaust plumbing and improving volumetric efficiency. But, eventually, all this effort comes down to the physical act of combustion. A crucial portion of this process requires placing the correct amount of fuel in the combustion chamber as efficiently as possible. In the time just after dinosaurs ceased to roam the earth, carburetors were the fuel-mixing device of choice. Then came mechanical fuel injection, followed by multipoint electronic fuel injection (MEFI), but even that is now considered rudimentary compared to injecting fuel directly into the combustion space.

GDI’s advantages are multi-fold. First, even with multipoint fuel injection, a certain amount of fuel ends up deposited on the walls of the intake port upstream of the intake valve. This fuel eventually contributes to combustion, but not necessarily at the proper time or in the best condition. The condition of fuel is as important as its ratio to the oxidizing air. Liquid fuel does not burn well; only vaporized fuel contributes to the combustion process.
An extreme example of this can be found in Top Fuel racing. Nitromethane is highly combustible, but nitro’s flame speed isn’t much different from gasoline. However, Top Fuel engines routinely employ ignition timing in excess of 50 degrees before TDC. The reason for this incredibly early ignition lead is that only 10 percent of the fuel in the chamber is vaporized. The rest remains a liquid. This is because these engines operate at very close to a 1:1 air/fuel ratio! The cylinder demands the spark light 50-plus degrees BTDC to initiate the burning process early enough to produce enough early heat to vaporize and burn the large amount of fuel in the chamber.

Gasoline engines operate with drastically leaner air/fuel mixtures, but the concept is still the same: only vaporized fuel will burn completely. With direct injection, the fuel can be introduced into the cylinder at pressures exceeding 2,200 psi so that a greater portion of the fuel will quickly vaporize. Even so, direct injection at very high pressures demands changes in the combustion space.

GDI engines generally employ a much different piston crown design than comparable non-GDI engines. The concept is to use a trough or depression in the piston crown that will direct the fuel once it has been injected. The focus of this trough is to aim a stratified or directed charge of a relatively rich fuel mixture at the spark plug to initiate the combustion process. Once ignition occurs, the remaining fuel can be combusted to produce an overall efficient mixture.

Generally, the high-pressure fuel injector is located near the center of the cylinder. Research indicates that late injection of the fuel into the cylinder is beneficial for emissions and fuel efficiency with the piston near TDC. A centralized trough in the piston crown tends to redirect the fuel spray upward toward the exhaust side of the chamber near the spark plug. This generates something the combustion research engineers call turbulent kinetic energy (TKE). A higher TKE tends to support improved thermal efficiency where more of the fuel is used in combustion.

There are several advantages to this approach. First, it reduces the chance for detonation since the fuel is concentrated more toward the center of the combustion space near the spark plug. Knock generally occurs from end gases with sufficient fuel that auto-ignite near the end of the combustion process. By concentrating the fuel around the spark plug, this greatly reduces the need for excessive ignition advance. By introducing the fuel microseconds before the required spark timing, pre-ignition is virtually eliminated because there is no fuel to pre-ignite!

The Aftermarket Approach
According to JE Pistons engineer Clayton Stothers, other than the piston crown design, there is not a significant difference in piston configuration between a forged GDI piston and one designed for carbureted or MEFI engines. Obviously, strength is a considerable concern to accommodate the higher cylinder pressures that will generate more horsepower.

One additional benefit of the proper piston top design is that more of the fuel is concentrated in the center of the combustion space, which prevents fuel from potentially being trapped near the cylinder’s outer circumference. Fuel that tends to collect around the outer edge of the combustion space often does not burn and therefore does not contribute to making power. These unburned hydrocarbons exit with the exhaust and contribute to reduced thermal efficiency and emissions.

GDI engines consistently reduce the amount of fuel that is trapped around the piston’s circumference, which—especially at part throttle—allows the engine to run at much leaner air/fuel mixtures, which improves fuel efficiency. The net result of this is there are examples of current GDI engines running at air/fuel ratios in excess of 30:1! For comparison, a standard MEFI gasoline rarely eclipses 14.7:1.

Because of the improved combustion efficiency, GDI engines can also run higher static compression ratios. As an example, GM’s LT1 takes advantage of the GDI design to push static compression to 11.5:1. JE’s EcoBoost V6 pistons (which are a turbocharged application) sport an impressive 10.0:1. Conventional multipoint EFI engines would not dream of running a turbo with that high of static compression on pump gas. Of course, the advantage to this higher compression is additional power as one full point of compression is generally accepted to deliver around 3 to 4 percent additional power for a normally aspirated engine.

Noted turbocharged engine builder Kenny Duttweiler is currently experimenting with an EcoBoost 2.3L engine and told HOT ROD he expects the engine could potentially make as much as 1,100 hp with a larger turbocharger.

So we can safely assume that GDI engines will continue to be the current trend in performance engines for the near future. Bosch projects that 20 percent of all production passenger-car engines will rely on gasoline direct injection by the year 2020.

This article originally appeared on hotrod.com

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