Directly speaking - a look at developments in GDI
by Matthew Beecham
Despite a slow start, gasoline direct injection (GDI) technology is becoming popular, representing the next weapon in the emissions battle.
Direct injection means injecting the fuel directly into the cylinder instead of premixing it with air in separate intake ports. That allows for controlling combustion and emissions more precisely. But it demands more advanced engine-management technologies.
On the face of it, GDI offers a potential 20% improvement in fuel economy and emissions compared with conventional gasoline engines. The original promises put forward for the first generation GDI systems, however, were certainly overstated. The fuel economy benefits that were touted at the time were largely based on a stop-go driving mode in Japan. However, in Europe and more so in the US, a lot of the driving situations simply don't lend themselves to getting this type of performance out of GDI. Also, there were some technical shortcomings from the first generation systems that, in addition to the fuel economy, did not really come through.
Since then, the technical focus on developing GDI has centered on reducing fuel consumption while boosting performance through homogeneous combustion. “In the old days, you had injection systems to increase the absolute power of an engine,” said Eduard Rikli, CEO, of the Swiss-based Mikron Technology Group “These days, however, that is still the case but a much larger proportion is covered by systems aimed at keeping fuel consumption and emissions as low as possible. We shall see that trend for years to come.”
On balance, the reasons for the slow adoption of GDI are many and varied although principally result from the economic balances and compromises which have to be made with the introduction of any new technology. “The vehicle manufacturers have to make considerable investment in significantly changed cylinder head designs, engineering capacity and the general infrastructure associated with a new technology,” said John Clack, commercial manager, automotive, Bosch UK. “In the US, despite the recent increases in crude oil prices, they have maintained a low fuel price policy. This has offered little incentive for the introduction of fuel saving technologies, whether GDI or diesel.
In Europe the emphasis has been on CO2 reduction and here, because of its higher thermal efficiency, diesel offers distinct fuel saving advantages. This, coupled with the fact that many European countries have fuel taxation strategies which greatly benefit diesel has resulted in a dramatic growth of diesel engined vehicles at the expense of gasoline.” However, Clack believes that forthcoming emission legislation could be the key for the future expansion of GDI technology. In the US GDI engines already comply with SULEV legislation with simpler exhaust gas treatment than is required with port fuel injection to the extent that costs for the overall system are on a par. In Europe, local air pollutant limits are the key. “If future legislation brings limit values in line for both diesel and gasoline then the likely requirement for expensive exhaust after-treatment on diesel could move the overall cost/benefit equation in favour of gasoline,” said Clack. In late 2005, Bosch launched its second generation gasoline direct injection systems.
Michael Crane, North America director of powertrain gasoline systems, Siemens VDO Automotive, believes that adoption of GDI technology in the US has lagged both Europe and Asia mainly because market drivers like fuel consumption and carbon dioxide have been stronger in those regions. He said: “With the advent of significantly higher fuel prices and potential CAFE increases in North America, OEMs are just now turning to direct injection as a viable means of reducing fuel consumption while simultaneously improving both engine performance and emissions.”
Crane argues that GDI is not a panacea for fuel economy, but has multiple advantages that make it very attractive under both current and projected market conditions. “Moving the fuel delivery point from the intake ports to the cylinder eliminates the hang-up or storage of fuel in the ports, promotes more precise fuel delivery, and allows an increase in compression ratio. These features combine - often in synergy with other engine improvements like turbocharging and downsizing - to deliver increased fuel economy, reduced cold start emissions, and better engine performance. GDI is unique in that the consumer realizes the best of all worlds with no compromises.”
Crane also reckons that there is little doubt that homogeneous or stoichiometric operation will be the roll-out strategy for GDI in North America, pointing out that VW/Audi, Lexus, BMW, and Mazda are all employing this strategy today and the first domestic GDI engine will be introduced later this year in GM's Pontiac Solstice. He said: “Stratified charge, lean-burn operation is feasible with GDI and can yield double-digit reductions in fuel consumption; however, this strategy requires advanced aftertreatment to control the excessive oxides of nitrogen that are produced under such conditions. The incremental on-cost and operating costs associated with the advanced after treatment required to meet the stringent regulatory requirements in the US make this an unlikely scenario for the next several years.”
In Japan, Mitsubishi Motors was the first to introduce GDI technology, launching it on the Galant/Legnum’s 4G93, which was later rolled out in Europe in 1998. In 1999, PSA Peugeot Citroen borrowed (under license) the GDI technology from Mitsubishi Motors and introduced a GDI engine although this was subsequently withdrawn from the market in 2001.
Osamu Fukasawa of Denso Corp’s powertrain management systems engineering department agrees that GDI technology has not spread rapidly although he reckons that the technology has the potential to outperform the port injection gasoline engine in terms of reducing fuel consumption, increasing engine output, and exhausting cleaner emission. However, Fukasawa believes that the biggest problem of GDI is high cost relative to port fuel injection systems. “GDI technology requires high pressure pump and high pressure injectors,” said Fukasawa. “Further, if GDI technology is used to reduce fuel consumption, NOx would be increased in the exhaust to necessitate the NOx removal system such as NOx catalyst, resulting in further increased cost. Also with using NOx catalyst, the sulfur contained in gasoline would become a factor. Stringent emission regulations for PM would be another reason to prevent GDIs to spread in the market.”
Bosch’s Clack agrees that GDI component cost is higher than for port fuel injection, but points out that it is possible to save cost in exhaust treatment systems such that the overall effect can be neutral.
Although GDI does not compete head-on with variable valvetrain, it is regarded as a complimentary technology. In comparing the cost of GDI with variable valvetrains, Dr Walter Piock, manager, advanced gasoline – innovation Centre, Delphi Powertrain Systems, told us: “It is difficult to give precise figures. As a guide, choosing a direct injection over a MPFI system means you must replace the injectors and the fuel rail and introduce a high pressure pump in addition to the feed pump from the tank. So the major cost driver here is the fuel pump. If you look to implement a new valvetrain system then you have to completely change the cylinder head and implement high precision parts to the different cylinders. As a very rough guess, compared to a sophisticated VVA [variable valve actuation], I would say that a homogenous direct injection gasoline system would cost slightly more than half.”
Piock also sees a number of synergies between GDI and diesel direct injection in terms of the injectors, pumps, sensing, and control software. He said: “The fuel system pressure is one order of magnitude different which requires different concepts and designs for gasoline and diesel injection systems. While modern passenger car diesel engines are dealing today with 1600 to 1800-bar fuel pressure gasoline direct injection systems are working with less than 200-bar. Homogenous systems with the main injection during the intake stroke have currently operating pressures up to 150bar. For stratified systems, operating pressures currently go up to 200-bar. In addition we have to consider the use of more corrosion and wear resistant materials and sometimes special coatings as gasoline does not have the same protective properties as diesel.
In addition to different pressures, some of the latest diesel injection systems deal with up to seven injections in one engine cycle in order to improve fuel economy and noise generation and to meet the emission regulations, i.e. two pre-injections, one main injection and two or three post injections. On the gasoline side of the business, however, clean and efficient combustion can be achieved without this level of control.” As far as the control software is concerned, Piock also sees some similarities between diesel and gasoline fuel injection systems in terms of fuel handling, control and cylinder balancing. “The handling and modeling of these systems is very similar,” said Piock.
Although GDI will continue to be a weapon in the technology armory of the OEMs in their fight to reduce CO2, Denso’s Fukasawa predicts that homogeneous GDI will be the mainstream technology with a 10% share in Japan by 2010. He said: “The key points would be to realize the high potential of GDI relating to fuel consumption, engine output and emission, while lowering costs. Stringent fuel consumption regulations would be another factor that would accelerate the growth of GDI.” In North America, Siemens VDO’s Crane forecasts that 2 – 5% of the North American gasoline engine market will use direct injection by 2010.
forecast market penetration could reach 12% in Western Europe by the end of this decade. However, if the fuel economy benefits can be truly demonstrated and GDI is proven to be a better solution over competing technologies, the potential market could be much higher.