auto maintenance January 10, 2018

Direct Injection – Huge Carbon Footprint!

In my automotive lifetime, there have been many major changes in how fuel is “fed” or delivered to the engine. From the turn of the last century right up to about 1980, almost all cars in the U.S. were “carbureted.” A carburetor is a device that uses the air pressure difference caused by engine vacuum to draw fuel out of the float bowl and atomize it into the air being drawn into the engine. Cheap and effective, and if we never had to worry about pollution or fuel economy we would probably still be using them. Various electronic controls were added to keep carburetors in service right up to the mid-eighties, but the speed of air flow, which is what a carburetor functions at, could never respond quickly enough to meet ever-tightening emission control and fuel economy requirements.

The conversion to various forms of “electronic” fuel injection occurred rapidly in the mid-eighties. First came central fuel injection: one or two large fuel injectors placed more or less where the carburetor had been. The upside was the engineering was rather simple and required few fundamental changes. The downside was that like a carburetor, central fuel injection sprayed the fuel at the entrance of the intake manifold. During the journey through the manifold a certain amount of fuel always falls out of the air, re-condensing on the walls of the intake manifold. Fuel will only burn when it is fully atomized in the air, so this fuel was lost, hurting fuel economy, and increasing emissions.

The next step was to move the point of fuel injection right up to the intake valve of the engine. Now the intake manifold was conducting only dry air, with the fuel being injected at the very end of the port, where the intake valve of the engine’s cylinders were. Thus multi-port or “port fuel injection” became the industry standard, through the mid-nineties right up to a few years ago. Port fuel injection required more injectors (one for each cylinder) and a higher pressure fuel system (30-65 pounds versus 3-10 for carburetors or central injection), but the boost in performance and economy were large while lowering emissions.

Further gains were achieved by electronically timing the injectors to only spray fuel when the intake valve of the particular cylinder was opening, often called “sequential fuel injection.”

The latest and most dramatic change has come about in the past five years with the mass introduction of “direct injection.” This moves the injector directly into the combustion chamber of the engine, a much more hostile environment. This is where the fuel and air burns, producing thousands of pounds of pressure and up to 2000 degrees of heat. The benefit is there is almost no time or place for fuel to fall out or re-condense in this arrangement. The downside is the fuel system must operate at much higher pressure (thousands of pounds) to overcome pressures in the cylinder, the components must be much stronger, and in the event of a problem, they are much harder to get to.

But as often happens in major design changes there was an unanticipated negative result: massive carbon buildups on the intake valves to such a degree that major engine problems are occurring on cars with as little as 50,000 miles. Valves so badly carboned up they will no longer close or seal properly, requiring major engine repairs or replacement.

It seems that the old arrangement with the injector outside the cylinder spraying on the intake valve had two great benefits. First, the cleaning agents in gasoline, both natural and added, did a wonderful job of cleaning the valve. Additionally, it cooled the intake valve, preventing carbon from cooking onto the valve like a frying pan in your kitchen.

Add to this the fact that most cars no longer have a replaceable fuel filter, leading to more particulate contamination in fuel. Again add to this the 10% ethanol mandate. Ethanol does not burn as efficiently as gasoline. And finally, the remorseless efficiency of extracting more gasoline from each barrel of oil has led to using heavier hydrocarbons from the bottom of the oil barrel, and it all adds up to a carbon-palooza.

This is usually the part where I cheerfully inform you of a new service or procedure that will alleviate this problem but I’m afraid I don’t have one yet. The automotive aftermarket is pretty good at rescuing the manufacturer from their foibles, but it’s still a work in progress. One manufacturer has actually added a set of fuel injectors back in the old place, spraying on the back of the intake valve to periodically clean them. A little desperate and not very cost-effective.

A cleaning machine that uses crushed walnut shells to scour the valves clean seems to offer the most hope, but as of now that procedure does require a certain amount of engine dis-assembly to use. In the meantime, all you can do is use high-quality fuel for what good it will do, and hope that if the problem occurs on your car it will be within the manufacturer’s warranty period. Hope, however, is not a good long-term strategy.

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