Introduction Single-cylinder optical engines are often used to study in-cylinder flow@ combustion@ and pollutant formation as part of combustion system development@ as well as providing validation data for simulations. When a single-cylinder engine is built@ it is not mechanically balanced@ as a multicylinder engine is. This can be compensated for with balance shafts or weights. In addition@ it is a cost-effective way to make use of the cylinder head from a multicylinder engine rather than a specially manufactured single-cylinder head. A multicylinder inline engine may also be modified for in-cylinder measurements by incorporating an extension block into one of the cylinders@ dispensing with the costly bespoke single-cylinder engine. In practice@ a four-cylinder direct-injection (DI) diesel engine was modified for incylinder flow@ mixing@ and combustion measurements by Hentschel et al. (1989) so that realistic gas dynamics could be reproduced.