An engine that generates power from fresh air is the ultimate automotive panacea. While the revolutionary Dearman engine doesn’t quite manage that, it’s likely to be as close as it will be possible to get – and one will be in a bus next year. Tim Deakin looks at the concept.
The UK’s bus manufacturers have become adept at reusing kinetic energy to cut fuel consumption, but they are yet to do the same with waste engine heat. Global technology firm Dearman, backed by public body Innovate UK and in partnership with Air Products and the MIRA proving ground, is in the process of unveiling something which may change that.
There is plenty of heat for the simple Dearman engine to recover and put to good use. “Around 60% of the energy in diesel fuel is wasted as heat,” says Deputy Chief Executive Michael Ayres. “Instead of disposing of it through the radiator, we want to do something useful with it.”
In a nutshell, the Dearman engine is fuelled by either liquid air or liquid nitrogen, stored in a tank on the bus. Both exist at a very low temperature and deliver the same effect. Within the Dearman engine’s combustion chambers, the liquid is combined with heat drawn from the adjacent diesel engine, causing it to expand by over 700 times as it converts to gas.
This expansion drives the pistons in the same manner as combustion does in a conventional engine, producing a turning motion which can be directed to the vehicle’s wheels, but without creating any harmful emissions at the point of use.
For now, the Dearman engine must work in conjunction with an internal combustion engine, which provides the waste heat required for the expansion process.
Dearman calls this application a ‘heat hybrid’. Its unit is coupled directly to the diesel engine’s crankshaft, and thus they drive through the same gearbox, simplifying the installation. Like a flywheel, it allows the diesel engine to play a lesser part in propelling the bus.
A central electronic control unit (ECU) is part of the set-up. It communicates with both engines’ individual ECUs to demand torque, but Mr Ayres does not rule out the future possibility of one ECU governing both engines to give further efficiency gains.
“What we’re trying to do is push the diesel engine into the background as much as possible while allowing operation within its most efficient range,” he says. “The central controller works out whether it is optimal to use diesel power, Dearman power, or both.”
In the beginning
The Dearman engine was first developed to provide zero-emission refrigeration in the food logistics sector, as a large cooling capacity is a by-product of its operation. It was then realised that in many countries, buses also require cooling, a need met by power-hungry air-conditioning units.
“Air-conditioning doesn’t just affect fuel consumption. It also adds weight,” says Mr Ayres. Dearman sees great promise for its engine in the Far Eastern double-decker markets, where up to 40% of fuel is accounted for by air-conditioning. It can offer a variant which just provides cooling.
“Once we had that technology in a bus application, we thought ‘what else can we do with it?’” he continues. “Diesel is likely to remain the dominant source of power in buses for a long time. Our engine, using waste heat generated during combustion, allows it to be used more efficiently.
“We asked ourselves whether we could develop a Dearman engine which could deliver a similar power output to a hybrid battery pack. Because our engine is made from aluminium, plastic and steel, we can produce it at a fraction of the cost, making it an attractive proposition.”
Payback time for the Dearman engine when used as a heat hybrid in an urban bus setting should be no more than three years, says Mr Ayres. It is expected to deliver around a 20% reduction in fuel consumption.
The Dearman installation will add a modest amount of weight, he says, but the company is working to recoup that via further efficiency gains.
It should ultimately allow downsizing of the diesel power unit, and when air chilling is required it can be provided as a by-product of the Dearman engine’s propulsion function, allowing weight to be removed. In other scenarios, saloon cooling can be provided at no cost.
The technical bits
In heat hybrid application, which will prove most attractive to UK operators, the Dearman engine is a four-cylinder unit developing around 20kW per litre of displacement. The manufacturer envisages a need for between 40-60kW of power, depending on the size of the bus.
It is simpler than an internal combustion engine, says Mr Ayres, requiring no cooling system of its own. Instead, it runs at room temperature, removing the thermal stresses encountered during the warm-up phase of a diesel engine.
Heat for the expansion process is harvested by passing the diesel engine’s coolant through a heat exchanger. This warms self-contained, re-circulated fluid that is injected into the Dearman engine’s combustion chamber at the same time as liquid air or nitrogen. The resulting air or nitrogen in gas form is vented to atmosphere.
Maintenance requirements are the same as for a diesel engine. Oil must be changed periodically, while the heat exchanger fluid level should be monitored.
Although its concept is uncharted territory for the bus industry, Mr Ayres accepts that the Dearman engine’s durability must match that of a diesel unit’s.
“Both engines share many wear characteristics but ours is without temperature swings. Also, because we’re not burning anything, there is no internal build-up of contaminants or deposits in the Dearman engine. We’re confident of being able to match an internal combustion engine’s durability,” he says.
Making the unit as easy to maintain as possible is also a priority. “Telling someone to put liquid air into their bus, and then providing them with something which requires a PhD to maintain, is not a sound business plan,” adds Mr Ayres.
Dearman has already engaged a number of manufacturers and operators to act in advisory and observational capacities, and it has received feedback on the unit’s maintainability. “Operators we are speaking to tell us that it will be easier for their engineers to maintain the Dearman engine than buses which involve high-voltage electricity,” he says.
“A Dearman engine is a piston engine. It has a block, head, crankshaft, and valve gear. Engineers have been familiar with those since buses were invented.”
Out on test
The Dearman engine in a bus application is approaching the validation-of-concept stage, where Dearman will prove that it can be integrated into a bus’ other systems. It has purchased a single-decker which will be used for testing at MIRA, expected to begin around the turn of the year.
“We have not yet arrived at the final design because we have developed the second generation of our smaller engine recently, and we want to ensure that all lessons from that are incorporated into the bus application,” says Mr Ayres.
“Our strategy is to bring the technology to the point where OEMs can confidently offer it to buyers in a new build. The heat hybrid testbed will validate its integration into a bus. Trials in the field and carrying passengers are likely by early 2017.”