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Gas direct-injection engines

Here’s a guide to the evolution and essential care of GDI engines.

Discussions of direct fuel injection are almost entirely focused on diesel engines. Increasingly, however, direct injection is being utilized in gasoline engines ranging from the one in the family car to pickups and now to smaller engines. Yet, gas engine fuel injection is completely different from the diesel pump-line nozzle systems.

The mid-1970s witnessed the introduction of a Bosch mechanical fuel-injection system on many European cars that were imported here. In the 1980s, GM and Ford introduced a hybrid fuel system called throttle-body injection (TBI). It consisted of a throttle plate(s) and an injector(s) above it. The fuel was introduced in an environment at atmospheric pressure.

Ford was the first to bring true fuel injection to the pickup truck market in 1986. That innovation was the port fuel-injection system (PFI). With this design, there is a fuel injector for each cylinder that is mounted at the junction of the intake manifold to the cylinder head (located approximately 100 mm or less than 4 inches from the intake valve). 

Port Inject Advantages

Whereas the General Motors TBI operated on low fuel pressure (9 to 13 psi), the Ford system ran at 45 psi. Once the industry switched over to PFI, the operating fuel pressure range was between 45 and 60 psi.

The need to meet stricter emissions and fuel economy standards brought the introduction of gasoline direct injection (GDI). 

With this design, a specially designed injector is placed in the combustion chamber of the cylinder head similar to that of a fuel injector in a diesel. This device injects gas directly into the combustion chamber at pressures up to 2,200 psi.

The benefit of GDI when compared with PFI is that there is no fuel lost in transit as the emulsified mixture travels through the intake runner of the cylinder head. The main advantage is in the phase change that occurs with the gas in the cylinder. Due to the latent heat of vaporization, this provides a cooling effect to the fuel-air mixture since heat is used to convert the liquid gasoline to a rarefied state.

The cooler charged air temperature from GDI then allows for the compression ratio of the engine to be increased in many applications to an almost diesel-like value of 14:1. The most effective method to improve fuel economy in an internal combustion engine is to increase the compression ratio.

Multiple Injectors

Yet, the evolution of these systems did not stand still. The latest trend is the combination PFI and GDI engine. With this design, the number of injectors is double the number of cylinders. There is a PFI injector at the cylinder head end of the intake manifold runner and an injector in the combustion chamber. The engine controller toggles back and forth between these two fuel injectors. 

Traditionally, the engine is fueled via the PFI injector when started or running at idle and at very light loads. During other operating states, the engine is supplied by the GDI system. There is not an operating state when both systems are being used simultaneously.

Problems with Valve Deposits

As many manufacturers do real-life testing before a design goes into production, they cannot duplicate every scenario that leads to GDI’s pitfalls. With GDI, there is no fuel wetting on the back side of the intake valve. Yet, during cam overlap, combustion gases are reverted there and turn into carbon deposits. These deposits tent-up on the intake valve and block airflow into the cylinder.

A phenomenon called low-speed pre-ignition (LPSI) can occur when the engine is cold or at intermediate temperature stages and driven at low rpm and under moderate loads.

In the best-case scenario, LSPI will just result in combustion noise or crack a spark plug. Yet, in many instances, it is so severe that it will take a ring landing off a piston or bend a connecting rod.

The series of events that can produce LSPI are too technical and in-depth to cover here. It has been determined that the engine oil is a huge contributor, though. It was found that engine oils with a high level of calcium are very prone to cause LSPI.

For this reason, it is imperative that if you are operating a GDI engine that you use the exact oil that the manufacturer specifies, which will be a low-calcium blend. General Motors, for example, labels its low-calcium oil as Dextros.

Taking Care of Valve Deposits

With the employment of a PFI/GDI system, the engine operating state with LSPI can be induced by the PFI system and is no longer a concern. Keep in mind that none of the automotive companies will be obligated to honor a claim for an engine destroyed by LSPI once the engine warranty is expired. 

If you have an engine that is GDI alone, you need to use a good-quality fuel system and injector cleaner to keep the pintle of the injector clean. However, due to the fuel being introduced in the cylinder bore, an in-tank cleaner will not remove deposits from the intake valve.

There are many excellent cleaning products now on the market that can be introduced into the induction system with the engine at a high idle speed that will work on loosening the valve deposits so they can be burned. 

It is a sound protocol to perform this service every 5,000 to 7,000 miles. Once the carbon gets too hard or thick, it will be impossible to remove it chemically. In that case, the cylinder head needs to be removed.

If the engine is a PFI/GDI system, its care doesn’t require much more than using the specified engine oil and a good in-tank fuel system cleaner every 3,000 miles. The chemistry will keep the valves clean along with the PFI and GDI injector pintles and make the system carefree.

Don’t be concerned to own a GDI-only engine. But I highly recommend you follow the maintenance steps I outlined here for years of trouble-free service. If not, you will probably be putting a new engine in the vehicle down the road.

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