Delphi Powertrain Study Concludes 3-Cylinder Turbo Gasoline Direct Injection Engine Offers a High-Value NOx and CO2 Solution for Cost-Sensitive Vehicle Segments
|CO2 reduction potential vs. OEM On-cost for turbocharged GDi and turbo-Diesel technologies to meet Euro 6 standards. Source: Kirwan et al. Click to enlarge.|
For the most cost-sensitive vehicle segments—e.g., compacts under 1,400 kg in weight—a stoichiometric 3-cylinder turbocharged gasoline direct injection (GDI) engine offers an optimal combination of cost, lower emissions and reductions in fuel consumption, according to an analysis by Delphi Powertrain presented at the recent SAE 2010 World Congress. (Earlier post.)
The Delphi study assessed the technology of downsizing and turbocharging; GDI fueling and its synergies with turbocharging; and an analysis of downsized 3-cylinder versus 4-cylinder engines. It also performed a value analysis comparing CO2 benefits for 3- and 4-cylinder gasoline and diesel powertrains.
|Turbocharged stoichiometric GDi engine schematic. Source: Kirwan et al. Click to enlarge.|
Vehicle electrification was outside the scope of the analysis, Kirwan et al. said, noting that they expected that both the widespread implementation of stop-start technology and hybrid vehicle growth would continue. These technologies complement diesel and gasoline powertrains, and their implementation does not alter the analysis or conclusions on the engines themselves.
Both downsizing (reducing total engine displacement) and downspeeding (reducing engine speed through changes to the transmission and/or final drive ratio) are effective methods to meet vehicle power needs with reduced fuel consumption.
For a given vehicle power requirement at constant speed, a downsized engine operates at increased BMEP (specific load), which results in greater overall efficiency and thus reduced fuel consumption. Maintaining a given vehicle power requirement at reduced engine speed also requires that the engine operate at higher specific loads, again resulting in greater overall efficiency and reduced fuel consumption.
The combination of the two, Kirwan et al. note, is particularly effective for reducing fuel consumption and CO2 emissions. The use of turbocharging and gasoline direct injection contribute to the delivery of sufficient torque across the engine and particularly at lower speeds in the downsized/downspeeded scenario. GDI operates in both homogeneous and lean stratified engine configurations.
Stoichiometric operation with homogeneous GDi allows the use of conventional 3-way exhaust catalysts and thus worldwide application without concerns for lean NOx production and aftertreatment.
Downsizing can be accomplished via reduction in the number of cylinders, and/or a reduction in the displaced volume of each cylinder. Delphi began its value analysis with a 1.6L naturally-aspirated baseline vehicle and evaluated the benefit of 25% (modest) downsizing to 1.2L turbocharged engines with downspeeding, looking at 4-cylinder naturally aspirated configurations versus downsized turbocharged 3-cylinder applications.
Overall, Delphi favors a 3-cylinder configuration as opposed to a 4-cylinder for reduced fuel consumption and regulated emissions for a number of factors, including better high load scavenging for high load performance. A 3-cylinder engine also offers cost and packaging advantages and provides acceptable NVH with counterbalancing. Delphi concluded that 3-cylinder turbocharged engines are attractive for engines up to 1.5 L displacement.
In its value analysis, Delphi found that for a compact vehicle meeting Euro 4 standards, a 4-cylinder turbo-Diesel reduces CO2 emissions at an on cost rate of €22 / percent CO2 reduction.
Our analysis of compact turbocharged vehicles configured to meet Euro 6 emissions in 2014 shows two gasoline configurations and one Diesel powertrain mechanization to have the best value. They reduce CO2 at an on cost rate of 24-25 euros / percent. The turbo-Diesel configuration is a downsized 3-cylinder engine with engine-out NOx that must be capable of meeting the Euro 6 NOx standard without lean aftertreatment. Both preferred turbocharged GDi applications are 3-cylinder stoichiometric engines with conventional three-way catalytic converters.
The turbocharged 3-cylinder stoichiometric GDi powertrains we considered noticeably reduce the CO2 emissions difference between gasoline and Diesel powertrains. And the stoichiometric GDi solutions have substantially lower cost, a factor that can be particularly important for lower-priced vehicles. The turbocharged 3-cylinder stoichiometric GDi solutions also employ a traditional 3-way catalytic converter to meet the most stringent NOx emissions regulations such as the US Tier 2 standards and future standards being postulated for Euro 6+. Thus turbocharged 3-cylinder stoichiometric GDi powertrains offer a robust method to reduce CO2 with low NOx for worldwide application in the compact vehicle segment.
—Kirwan et al.
John E. Kirwan, Mark Shost, Gregory Roth and James Zizelman (2010) 3-Cylinder Turbocharged Gasoline Direct Injection: A High Value Solution for Low CO2 and NOx Emissions (SAE 2010-01-0590)