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What is Direct Injection and what are the advantages/disadvantages?

Posted on April 16, 2013

Direct Injection, sometimes referred to as “DI”, is a fuel-efficiency and power-enhancing technology offered by many manufacturers. It is touted as a benefit and desirable feature, and is becoming ubiquitous in luxury and high-performance models. DI does indeed offer certain advantages, although it has proven troublesome in many manufacturers’ implementations. If you’re interested in the technical aspects, a DI system injects fuel directly into the engine’s combustion chamber, as opposed to further upstream, before the intake valve, as in a traditional port injection engine. This allows the fuel spray to cool the charge in the combustion chamber, allowing a higher compression ratio, as the cooling effect staves off detonation. The higher compression ratio directly results in more torque and power, usually made available throughout the engine’s rev range. The increased torque and power mean that for any given acceleration, you’re easier on the gas pedal, meaning that fuel economy improves. These improvements are real and perceptible to the driver, versus may gimmicks that claim to improve power, but don’t in the rev range where most driver’s operate. The one major downside of DI however, is that in many, perhaps all, cases, it results in carbon-buildup / coking on the intake valves. In a port injection engine, the incoming fuel-air mixture flows over the intake valves, and the detergent in fuel washes any deposits on the valves. Due to emissions regulations, the intake valves of all engines are exposed to considerable oil/carbon contaminants, as the blow-by of combustion is rerouted into the intake tract via the positive crankcase ventilation (PCV) system. In a DI engine, the fuel no longer washes over the intake valves, as it is injected directly into the combustion chamber, allowing the oil/carbon contaminants to build up on the intake valves in a thick layer. The inevitable carbon-buildup / coking results in severe degradation of engine performance as the valves are no longer able to operate as necessary and eventually the check engine light is triggered (meaning that the car will no longer pass emissions tests). Contrary to what many laymen or even mechanics say, in DI cars effected by carbon-buildup / coking, using premium or “top-tier” gasoline will do nothing to alleviate the problem, as this gasoline never touches the effected valves. The duration of time it takes for significant and deleterious amounts of carbon-buildup / coking to occur varies greatly depending on model and driving style (amongst other factors), but it can be as short as 15,000 miles of driving. Once a car is affected, the only solution is a costly and time-intensive manual cleaning of the valves, via either direct scraping or walnut-shell blasting. Many of the German manufacturers, which were the first to venture into DI technology, have experienced severe carbon-buildup / coking issues. They have employed a variety of courses of action, ranging from denying the problem or blaming the customer for using low-quality gas to taking on the cost of an expensive manual cleaning. In addition to the German manufacturers, more recently the American, Japanese and Korean manufacturers have employed DI on their cars. Reports of problems are difficult to asses, as the problem can take anywhere from one to three years to manifest itself. In terms of a design solution, Toyota / Lexus has clearly shown that a duel-injection configuration, one employing both port and direct injectors, keeps the intake valves clean and problem free and provides all of the performance advantages of a DI system. However, a duel-injection configuration is more expensive compared to a traditional DI system and very few models use this duel-injection configuration. Rumors have been circulating that more manufacturers have such problems in the works. Certain manufacturers claim to have solved the carbon-buildup / coking problem through careful filtering of the PCV system and/or special timing of the fuel injection events. The effectiveness of such solutions is not yet clear, but Car Match is monitoring reports of problems with such engines closing. It is clear that DI is here to stay, as it is a convenient way for manufacturers to meet more stringent emissions / fuel economy regulations while providing good power. As with all technologies, it is likely to mature and become more reliable. Until that time, Car Match will advise you in your report as to which cars are likely to develop DI related problems and whether you are likely to experience such problems.

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