A combined thermodynamic cycle based on methanol dissociation for IC (internal combustion) engine exhaust heat recovery

• A combined thermodynamic cycle using methanol as working medium for IC engine exhaust heat recovery is proposed.
• The external bottom cycle of exhaust heat recovery and IC engine working cycle are combined together.
• IC engine fuel efficiency could be improved from both in-cylinder working cycle and external bottom cycle.
• The maximum improvement to the IC engine global fuel efficiency reaches 6.8 percentage points at full load.

In this paper, a novel approach for exhaust heat recovery was proposed to improve IC (internal combustion) engine fuel efficiency and also to achieve the goal for direct usage of methanol as IC engine fuel. An open organic Rankine cycle system using methanol as working medium is coupled to IC engine exhaust pipe for exhaust heat recovery. In the bottom cycle, the working medium first undergoes dissociation and expansion processes, and is then directed back to IC engine as fuel. As the external bottom cycle and the IC engine main cycle are combined together, this scheme forms a combined thermodynamic cycle. Then, this concept was applied to a turbocharged engine, and the corresponding simulation models were built for both of the external bottom cycle and the IC engine main cycle. On this basis, the energy saving potential of this combined cycle was estimated by parametric analyses. Compared to the methanol vapor engine, IC engine in-cylinder efficiency has an increase of 1.4–2.1 percentage points under full load conditions, while the external bottom cycle can increase the fuel efficiency by 3.9–5.2 percentage points at the working pressure of 30 bar. The maximum improvement to the IC engine global fuel efficiency reaches 6.8 percentage points.