The in-line three-cylinder gasoline engine of the new "N-Box" light car, which Honda Motor Co Ltd released Sept 1, 2017, has a long-stroke design.
The ratio of its stroke to its cylinder bore (inner diameter) is as large as 1.29. The stroke/bore ratio is the world's highest level for a mass-produced engine.
The bore x stroke of the new engine, "S07B," is 60.0 x 77.6mm. And the bore x stroke of the previous model, "S07A," is 64.0 x 68.2mm (stroke/bore ratio: 1.07). Recently, long-stroke designs are frequently employed, but their stroke/bore ratios are usually up to about 1.1. The ratio of 1.29 is much larger than the stroke/bore ratios of other companies' engines.
Honda increased the ratio to 1.29 to strengthen the longitudinal vortex (tumble) of airflow in the cylinder. As a result, fuel can be well mixed with air, improving combustion speed.
When the engine rotation speed of the new engine is the same as that of the previous engine, the longer-stroke cylinder can realize a higher piston speed. The higher piston speed increases the speed of airflow, making it possible to strengthen the tumble.
Also, with the longer stroke, the workload of the piston per combustion can be increased, making it easier to improve thermal efficiency.
On the other hand, with a long stroke, it is difficult to increase rotation speed, often resulting in a lower output. This time, Honda employed the "VTEC" variable valve lift mechanism on the suction side for the engine of a light car for the first time, making it possible to prevent output from lowering.
The company increased the lift of the suction valve in the engine rotation speed range from about 4,500rpm so that a large amount of air can be taken in. The previous engine uses the "VTC" variable valve timing mechanism.
Compared with the previous N-Box, Honda increased the fuel efficiency of the naturally-aspirated model from 25.6km/L (approx 60.2mpg in the US) to 27.0km/L and that of the turbo engine model from 23.8km/L to 25.6km/L.