Mazda hasn't been shy about its dedication to the internal combustion engine. After all, the Japanese company will likely arrive first to market with a homogeneous charge compression ignition engine (HCCI), which will provide tremendous fuel economy improvements without the loss of performance.
But today we're talking turbo lag, and how Mazda aims to reduce it. Jason Fenske of Engineering Explained details how the automaker accomplishes just that. Specifically, Jason takes a look at the 2.5-liter turbocharged inline-4 found in the Mazda CX-9 and Mazda 6.
First and foremost is the exhaust manifold. It's short, so the exhaust gas holds on to as much energy as possible as it travels through various pipes. The gas is, ultimately, heading to the turbocharger. The second part of the process to keep turbo lag at bay is the valve design. Jason says it's similar to a variable geometry turbo, but the components are built into the exhaust manifold. Below 1,620 rpm, the valve closes and becomes more constrictive for the exhaust flow.
The process increases the velocity of airflow to spool up the turbocharger quickly, though it sacrifices efficiency. The engine also runs a 10.5:1 compression ratio to help make up for some lost efficiencies. Above 1,620 rpm, the valve opens up to allow for higher gas flow and the need to spool up the turbo more quickly isn't as necessary.
Finally, the cylinder firing strategy and 4-3-1 exhaust help the process. In a 4-cylinder engine, the firing order performs as 1-3-4-2. Mazda chose to split the exhaust in a unique manner to maxmize scavenging. When the exhaust valve opens, peak pressure and energy occur. The process forces the exhaust gas out, but some gas remains at a lower pressure. A suction effect to pull out the remaining gas at the end of the stroke would be ideal, and that's what Mazda has done.
The cylinder next to the other (in this example its cylinder one beside cylinder two) creates the suction effect to pull out low-pressure exhaust gas. High pressure and low pressure are always beside each other to help one another evacuate the exhaust gases.
It's complicated, so see the entire explanation in the video right up above.