Mazda engine engineers have apparently upset the common wisdom about basic features with its new gasoline technology (termed Sky-G) and diesel technology (Sky-D). The old wisdom had it that diesels must have high compression ratios (C/R) to facilitate engine warm up and high energy efficiency while Otto engine C/R for normal street vehicle use has been assumed to be limited due to inherent combustion knock.
Mazda has relegated these assumptions to the history books by cutting diesel C/R to the unheard of low level of 14:1 for automotive use while increasing the C/R of the gasoline engine to the equally unheard of level of 14:1(13:1 U.S.) for mass market street vehicles. The initial diesel is 2.2L and gasoline engine is 2.0L
A game changing achievement, for the Sky-D engine is complete elimination of diesel exhaust out NOX after treatment yet within U.S. NOX emissions requirements tier 2 bin 5. A coincidental advantage is early fuel injection which provides more time for power generating expansion. Variable lift exhaust valves retain high temperature during the compression stroke to insure fuel ignition during warm up at the new low C/R level. Combustion temperature after warm up, however, is below the level that forms NOX, aided by cooled low pressure exhaust gas recirculation EGR(U.S.).
The lower C/R increases Mazda’s Sky-D efficiency to 20% better than its current diesel aided by reduced friction and thermal losses. Friction reduction is said to be 15%. The lower C/R has a further advantage of reduced engine structural strength requirements (lower piston and rod weight) with related cost and total weight cuts.
A new ceramic diesel particulate filter (DPF) is said to be little needed due
to increased injection time to avoid locally rich soot producing areas.
12 hole piezoelectric injectors deliver fuel to combustion chambers at 2900 psi. with 2 to 8 injections per cycle. (more)
The Sky-D engine employs twin sequential turbochargers that effectively eliminate initial boost lag. Torque is 310 ft/lb at about 1800 rpm. Red line RPM of 5200 is remarkably high for a 2.2L diesel producing 173 HP @ 4500 RPM.
Mazda’s Sky-G 2.0L direct injected gasoline engine is no less dynamic with over 18% fuel efficiency gain from its also game changing 14:1(13:1 U.S.) C/R. This is made possible by unique unusually long exhaust manifold tubes tuned to optimize pulsed extraction of exhaust from the combustion chamber. The result of this feature is much reduced residual exhaust temperature in the engine thereby cutting the conditions for knock. The 2.0L Sky-G is rated 151 ft/lb at 4000 rpm
Mazda’s new Sky-D & G engines realize further gains from longer stroke and refined piston cavity configurations. Both engines are mated to 6-speed
early lock up automatic transmissions and 6-speed manual transmissions which have reduced throw shifters. Mazda vehicles with these engines will be sold worldwide.
Mazda’s new engines are scheduled for first use in vehicles in late 2011 and 2012. Both D & G engines are based on many common parts made on common tooling. Future Sky G engines will include 1.3L & 1.5L sizes followed by a boosted 2.0L Sky- G engine to replace V6 engines. The firm’s 7 year goal is 30% gain in fleet fuel economy.
Taken together, Mazda has likely up ended many plans of auto industry power plant engineers and provided another testimonial to the widely held belief that the age of internal combustion engines has a long way to go. That significant fundamental gains in both Otto and Diesel technology would emerge simultaneously from one firm and share many key common parts and tooling, is remarkable. If the combination of Sky-D engine cost and fuel
efficiency begins a new age of diesel cost/effectiveness, fuel planners could have a new dynamic to consider
Mazda’s demonstrated 18% fuel economy improvement with its high compression Sky G gasoline engine is significant contrasted with industry efforts to use downsized turbocharged engines to replace larger displacement naturally aspirated engines. Limited gains with some of the downsized turbo engines are attributed to the need for lower compression in the smaller engines to offset turbo induced knock. In one example, highway MPG has been increased by only 3% vs. a larger non-turbo engine that has a higher C/R.
An unanswered question at release of this report is whether Mazda’s unique Sky-D diesel combustion system which meets tier 2 bin 5 emissions limits may be able to perform within tier 2 bin 2 level cost/effectively. Although Sky-D engine combustion is said to be outside the conditions that form NOX, more development work overall may be needed to meet bin 2.