Well-known are Atkinson and Miller cycles. These are cycles with varying compression and expansion ratios. Application of these cycles makes it possible to use working gas energy to a fuller extent.
Suggested herein is a different operation mode of a Diesel engine with varying compression and expansion ratios, when the exhaust valve briefly opens and then closes during the compression stroke. The operation is explained in the following drawings:
Fig.1 – the intake stroke;
then the compression stroke starts (Fig.1b);
further on, the exhaust valve is partly opened with air bleeding from the cylinder into the exhaust path (Fig.1c);
then the exhaust valve closes and the second compression phase starts: fuel is injected into the head-end volume (Fig.1d), ignition and power stroke follow;
the cycle completes with the exhaust stroke (Fig.1e).
Brief opening and closing of the exhaust valve during the compression stroke can be imple-mented with the help of an additional cam lobe. Opening the exhaust valve during compression allows some of the air from the head-end volume of the cylinder to escape into the engine’s exhaust system. This makes it possible to:
* reduce pressure in the cylinder’s head-end volume during compression, which ensures lower actual air compression ratio as compared to geometric. At that, the compression ratio will be lower than the expansion ratio, providing a possibility to utilize the energy of expanding gas more efficiently and thus improve engine’s overall performance.
* change actual engine compression ratio during operation (different optimal compression ratios are required for different engine operation modes).
* increase oxygen concentration in exhaust gases, making it possible to burn black carbon directly in the exhaust system and thus lower the total content of black carbon in exhaust gases.
Let’s look at the mechanism to reduce nitrogen oxide and black carbon emissions. Conditions in the combustion chamber are configured so as to allow the least possible amount of nitrogen oxides to be generated. Black carbon produced in the process is burned directly in the exhaust manifold in the presence of high temperature exhaust gases and increased amount of oxygen provided by air bleeding into the exhaust manifold during the compression stroke.
It’s plain to see that working gas pressure at a given point in the crankshaft rotation cycle before the heat generation starts depends on residual gases pressure in the exhaust manifold and exhaust valve closing timing during compression cycle.
Depending on the ICE operation mode, the working gas pressure in the cylinder at exhaust valve closing timing on compression stroke will change from the maximum (max power) to minimum (idle). Simultaneously, initial pressure of the second compression phase will change, too, resulting in the changing average values of compression and power strokes.
Higher pressure values of these strokes not only increase the output power but also lead to higher friction losses in ICE crank gear friction couples. Lower average pressure of compression and power strokes during no-load or part-load operation will result in reduced mechanical losses and enhanced ICE fuel efficiency.