MILLER CYCLE

       A high turbocharger efficiency contributes to reduced carbon dioxide emissions by improving the engine efficiency. The same turbocharger, combined with a ‘smart’ turbocharging system able to guarantee an optimum air/fuel ratio under all conditions, would contribute even more to the control of soot emissions. NOx is produced during combustion at very high temperatures—something which can hardly be influenced by changes in the turbocharging system since the flame temperature depends on local conditions in the cylinder and not on the global mean air/fuel ratio.

Hence efforts were put-in to make the combustion chamber efficient in reducing combustion air temperature which consequently reduces NOx emissions. Thus, a joint development programme involving both the turbocharging system and the engine, aimed at reducing the temperatures of the working cycle in the cylinders.

            The first idea was turbocooling, in which the charge air is cooled in a process that makes use of a special turbocharger. If the pre-compressed air is further compressed in a second-stage compressor, then cooled and expanded through a turbine, very low temperatures can be obtained at the cylinder inlet. First evaluations revealed that the available turbocharger efficiencies for this process were not high enough for reasonable engine efficiencies, ABB Turbo Systems reports.

The Miller cycle promises much better results.

MILLER CYCLE

The idea is similar to that on which turbocooling is based. The charge air is compressed to a pressure higher than that needed for the engine cycle, but filling of the cylinders is reduced by suitable timing of the inlet valve. Thus, the expansion of the air and the consequent cooling take place in the cylinders and not in a turbine. The Miller cycle was initially used to increase the power density of some engines (see Niigata engines).

Reducing the temperature of the charge allows the power of a given engine to be increased without making any major changes to the cylinder unit. When the temperature is lower at the beginning of the cycle the air density is increased without a change in pressure (the mechanical limit of the engine is shifted to a higher power). At the same time, the thermal load limit shifts due to the lower mean

temperatures of the cycle.

Promising results were obtained on an engine in which the Miller cycle was used to reduce the cycle temperatures at constant power for a reduction in NOx formation during combustion: a 10 per cent reduction at full load was achieved, while fuel consumption was improved by around 1 per cent. This was mainly due to the fact that with the Miller cycle—at the same cylinder pressure level—the heat losses are reduced due to the air/fuel ratio being slightly higher, and the temperatures lower.

Thus, the reduced temperature of combustion will reduce NOX emissions.

HOW

            During the intake cycle of the combustion, the inlet valve is closed before its normal closing time. This expands the air and helps in reducing the temperature.

            Imagine, the piston is moving from TDC to BDC with its inlet valve open. The air draws in as the piston goes down towards BDC. Consider that I closed inlet valve 20degrees before BDC, the further movement of the piston for this 20degrees will expand the air. This expansion reduces the air temperature which is already drawn in.