The pressure waves in the exhaust manifold of a diesel engine can have a significant effect on the performance of the engine and its exhaust after-treatment devices. The pressure wave actions were investigated in the exhaust system of a single cylinder diesel engine through both experimental and simulation methods. The characteristics of the exhaust pressure waves under different engine operating conditions, such as engine load, exhaust back pressure, and injection strategies, were examined in this work. Pressure measurements were made at multiple locations in the exhaust system. The results showed that the strength of the exhaust pressure wave was affected by both the in-cylinder pressure and the back pressure in the exhaust system during the period when the exhaust valves were open. The exhaust gas flow velocity was also estimated by the one dimensional simulation tool AVL BOOST. The results suggested that the velocity of the exhaust gas fluctuated during the engine cycle, and followed a similar trend as the exhaust pressure wave. The transient gas flow velocity was high when there was a strong compression wave, and it was reduced when the pressure fluctuations in the exhaust manifold were small. A shock tube test bench was setup to complement the empirical work performed on the engine system. Optical methods were used to study the effect of the pressure wave and the accompanying high velocity gas flow on the atomization and distribution of liquid spray from a low pressure fuel injector. Such an injector is typically used in a diesel exhaust after-treatment system. From the results, it was seen that a transient high velocity gas flow had a significant effect on the spray development.