Abstract
The exhaust gas of heavy duty diesel engines can provide an important heat source that may be used in a number of ways to provide additional power and improve overall engine efficiency. The sizing of a heat exchanger that can manage the heat load and still be of reasonable size and weight without excessive pressure drop is of significant importance especially for truck applications. This is the subject of the present work. To approach the problem, a total of five different configurations are investigated and a comparison of conventional and state of the art heat transfer enhancement technologies is included. Two groups of configurations are examined: (a) a classical shell and tube heat exchanger using staggered cross-flow tube bundles with smooth circular tubes, finned tubes and tubes with dimpled surfaces and (b) a cross-flow plate heat exchanger, initially with finned surfaces on the exhaust gas side and then with 10 ppi and 40 ppi metal foam material substituting for the fins. Calculations were performed, using established heat exchanger design methodologies and recently published data from the literature to size the aforementioned configurations. The solutions provided reduce the overall heat exchanger size, with the plate and fin type consisting of plain fins presenting the minimum pressure drop (up to 98% reduction compared to the other configurations), and the 40 ppi metal foam being the most compact in terms of size and weight. Durability of the solutions is another issue which will be examined in a future investigation. However, coupling of the exhaust heat exchanger after a particulate trap appears to be the most promising solution to avoid clogging from soot accumulation.
Highlights
Despite recent improvements of diesel engine efficiency, a considerable amount of energy is still expelled to the ambient from the exhaust gas
An Organic Rankine Cycle System (ORCS) was installed on a Mack Truck diesel engine and the lab test results revealed an improvement of bsfc of 10-12%, which was verified by highway tests
If the standard plate and fin type heat exchanger is compared with the shell and tube heat exchanger with finned tubes, there is a significant (~66.5%) advantage in terms of volume, but the mass is increased by 20.6%
Summary
Despite recent improvements of diesel engine efficiency, a considerable amount of energy is still expelled to the ambient from the exhaust gas. This, depending on engine operating conditions, is in the order of 30-40% of fuel consumption For this reason, considerable improvement of diesel engine bsfc can be achieved through the utilization of exhaust gas energy and a number of technical solutions and possibilities have been proposed and are currently under investigation by research institutes and engine manufacturers. The installation of a Rankine Cycle is considered as a feasible solution for efficiency improvement in heavy duty diesel engines for trucks [6], [7] and for smaller applications such as passenger cars [8]
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