As you know there is some exhaust back pressure with a major part being push-back (resonance) from the other cylinders. To overcome this problem, Mazda played on the idea of long tube tuned exhaust headers. Hot-rodders have used tuned headers to help extract the exhaust by making each tube length a specific length so one cylinder’s exhaust pulse helps extract another’s. They measured the timing of each exhaust pulse to determine the needed length to prevent push back at low to mid rpm range. The exhaust tubes needed to be 600 mm (about 23.5 inches) long between cylinders on opposite strokes to reduce exhaust gas push-back. To fit the exhaust manifold in a reasonable space, they used a wrap-around Y-type tube design to get the needed length between two cylinders. Then the two tubes run into one pipe dumping into the catalytic converter, leading to the 4-2-1 design. This cut the residual cylinder gases from 8 to 4 percent, thus lowering the cylinder temps to help defeat engine knock.
Murphy’s Law showed up with the 4-2-1 design. The engineers now faced a major issue with the catalytic converter’s light-off time due to the distance from the exhaust ports. They adapted another old idea of retarding ignition timing to increase exhaust gas temps to heat the catalytic converter fast enough to prevent any increase in exhaust emissions. But this causes cold run misfires, so they made a deep cavity in the piston specifically shaped so the air and fuel are directed toward the spark plug creating a stratified charge (rich enough for combustion) around the spark plug. The specially shaped piston cavity had a benefit Mazda took advantage of which prevented the combustion flame from coming in contact with the piston causing combustion cooling loss, which would prevent flame growth. Another trick Mazda used was to map the engine for the shortest time the air and fuel spend in the chamber before ignition and intensify air flow turbulence in the chamber for better mixing.
All of these combustion chamber changes allowed them to reduce the weight of the pistons by 20 percent, reduce connecting rod weight 15 percent, and reduce piston ring drag 37 percent. The engineers didn’t stop there; valve train friction losses were next on the list. The engineers used another hot-rodder trick of roller rockers in the form of roller followers on the cam. Mazda still was looking for more energy reduction, so it reduced the oil pumping losses 45 percent by electronically controlling the oil pump. All of the weight and friction reductions allowed Mazda engineers to increase fuel economy by 15 percent and add 15 percent low to mid range torque over previous models.
SKYACTIV- D (Diesel Engines)
Mazda’s engineers defied conventional wisdom by lowering their diesel engine to a 14:1 compression ratio. This lowered emissions enough to do away with expensive exhaust after-treatments like Selective Catalyst Reduction (SCR) or Lean NOx Trap (LNT), but increased performance and fuel economy. This idea and some other improvements give the Mazda diesel 20 percent better fuel mileage while meeting Euro Stage 6, Japanese Long Term, and US Tier2 Bin5 emissions. Obviously, lowering compression ratio equals lower pumping losses, but this also allowed Mazda to optimize injection timing for better air/fuel mixture combustion while lowering engine weight and rotating mass.
Obviously lowering compression ratios reduces combustion heat thus lowering NOx. This also allows more advanced injection timing giving the air and fuel molecules more time to better mix before combustion. This does two things. First, it lowers combustion flash temperatures which lowers NOx and the better mixing gives a more complete burn greatly reducing particulate matter (PM or soot). Most diesels delay injection timing so the piston is starting down to lower emissions but Mazda knew combustion close to TDC makes for a more efficient engine.
It went after that expansion ratio, which is greater at top dead center (TDC). But again, if you change something usually other problems arise and it did. Lower compression causes lower heat so how do you start and idle a cold diesel with low compression plus get any power for acceleration? The answer required several new ideas like using multi-hole piezo injectors capable of rapid response times (durations in the 300 micro-seconds) and 9 injections per combustion stroke.