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Dearman liquid air engine moving into performance mapping, in-vehicle trials; diesel hybrid potential

The Dearman liquid air engine—an innovative heat engine that uses liquid air (or liquid nitrogen) as a “fuel” and emits cold air as exhaust (earlier post)—completed its shakedown testing milestone at the end of 2013 at Imperial College, London, and is moving into a three-month program of tests and performance mapping.

The developer, Dearman Engine Company (DEC), confirms that the engine remains on track for integration and installation on a vehicle by MIRA (Motor Industry Research Association) in the first half of this year. The project—in partnership with MIRA, Air Products and Loughborough University and jointly funded by the consortium partners and the UK Government (IDP8)—will demonstrate and test the Dearman Engine on a refrigerated truck providing zero-emission cooling and power during 2014, before moving to full on-road field trials.

The Dearman Engine operates by the vaporization and expansion of cryogenic fluids. Ambient or low grade waste heat is used as an energy source with the cryogen providing both the working fluid and heat sink. The Dearman Engine process involves the heat being introduced to the cryogenic fluid through direct contact heat exchange with a heat exchange fluid (HEF) inside the engine.

Prior cryogenic expansion engines have worked on an open Rankine cycle—i.e., similar to a traditional steam engine but operating across a different temperature range. In this approach, the cryogenic fluid is pumped to operating pressure and vaporized through a heat exchanger, before expansion in the engine cylinder.

This approach has a number of drawbacks, DEC suggests, as the heat exchanger must be large to cope with the heat transfer rates and heavy to withstand the high pressure. Additionally, little heat transfer occurs in the expansion stage (near adiabatic expansion) reducing the work output.

The Dearman Engine instead uses the heat exchange fluid to facilitate extremely rapid rates of heat transfer within the engine. This allows injection of the liquid cryogen directly into the engine cylinder whereupon heat transfer occurs via direct contact mixing with the HEF. The heat transfer on injection generates very rapid pressurization in the engine cylinder.

A Dearman Engine power cycle.

Direct contact heat transfer continues throughout the expansion stroke giving rise to a more efficient near-isothermal expansion. With the pressurization process taking place in the cylinder, the amount of pumping work required to reach a given peak cylinder pressure is reduced.

After each expansion cycle the heat exchange fluid is recovered from the exhaust and reheated to ambient temperature via a heat exchanger similar to a conventional radiator.

The Dearman engine is constructed almost entirely from the components of a conventional piston engine, requires little maintenance and has a light environmental impact.

The Dearman engine could be used in a number of configurations: on its own, as the prime mover of a zero emissions vehicle (ZEV); combined with an internal combustion engine (ICE) to form a ‘heat hybrid’; or as a power-and-refrigeration unit.

Dearman Engine IDP8 Refrigerated Van
  Vehicle manufacturers and industrial gas producers have begun to offer vehicle refrigeration for lorries and trailers based on the evaporation of liquid nitrogen, but these systems do not extract any power from the evaporation process. The Dearman engine extracts both shaft power and cold from the same unit of liquid air or nitrogen.
First the cryogen is vaporized in a heat exchanger in the refrigeration compartment, so cooling it down; then the high pressure gas is used to drive the Dearman engine, the shaft power of which can be used to drive a conventional refrigeration compressor or for auxiliary power.

The engine, designed to provide the power for refrigerated trailer applications, could be in production within two years and, with a network of industrial gas plants across the UK already producing liquid nitrogen, there is no infrastructure barrier to rapid deployment, the company said.

The concept for the new technology includes a diesel hybrid application. By harnessing the low grade waste heat of the internal combustion engine cooling loop, the Dearman engine can deliver 25%+ reduction in fuel consumption for a diesel heavy duty engine. The ability to work alongside other waste heat recovery systems is an additional advantage. Further development work is underway in this area.

Preliminary findings from a new report on the use of liquid air engines in commercial applications from the Liquid Air Energy Network (LAEN) (earlier post), Centre for Low Carbon Futures (CLCF) and University of Birmingham, to be published in early March, suggest that the adoption of liquid air technologies in heavy-duty vehicles could reduce the UK’s diesel consumption by 1.3 billion liters (343 million gallons US) and its carbon emissions by over a million tonnes by 2025.

Transport refrigeration
Transport refrigeration today is overwhelmingly powered by diesel. The Transport Refrigeration Unit is compressor-driven either from the vehicle’s main engine, or, on larger trucks and trailers, by a secondary “donkey” engine. Refrigeration can consume as much as 20% of a refrigerated vehicle’s fuel, causing CO2 emissions of almost 50 tonnes per vehicle per year from refrigeration alone.
Donkey engines are less strictly regulated than main drive engines, which means they typically emit high levels of nitrogen oxides (NOx) and particulate matter (PM). Comparing regulatory standards suggest that a trailer refrigerator engine emits six times more NOx and 29 times more PM than a Euro 6 truck engine.
On this basis, the adoption of liquid air on just 30% of Britain’s refrigerated trailers would reduce emissions of NOx by more than 1,800 tonnes, equivalent to taking almost 80,000 Euro 6 trucks off the road, and eliminate 180 tonnes of PM, equal to removing 367,000 such trucks from service.

The engines could also reduce local air pollution significantly: introducing liquid air trailer refrigeration alone would cut emissions of carcinogenic particulate matter by 180 tonnes per year, equivalent to taking 367,000 modern diesel trucks off the road.

The report has also identified that the roll-out of liquid air vehicles could be fueled entirely from existing spare industrial gas plant production capacity until at least 2019.

The engine is the brainchild of archetypal British garage-inventor, Peter Dearman, and subsequently developed in partnership with top UK engineering consultancy, Ricardo, and a number of leading UK Universities including Leeds, Birmingham, Loughborough and Brighton.

Liquid air as a new zero-emission energy vector emerged into public view in May 2013 with a report from the Centre for Low Carbon Futures (CLCF) entitled “Liquid air in the energy and transport systems: Opportunities for industry and innovation in the UK,” launched at a conference hosted by the Royal Academy of Engineering. Contributors to the nine-month study included National Grid, Arup, Ricardo, Messer Group, Spiritus Consulting and academics from the Universities of Leeds, Birmingham, Strathclyde, Brighton, Queen Mary University of London and Imperial College.

The CLCF report found that liquid air could reduce diesel consumption in buses or freight vehicles by 25% using a liquid air / diesel hybrid, while using a liquid air engine would cut emissions from refrigeration on food trucks by 80%. The report also raised the possibility of zero-emission liquid air city cars filling up at road-side forecourts at a fraction of current fuel costs and with lower lifecycle vehicle emissions than either electric or hydrogen powered vehicles.

Liquid air offers significant potential benefits as a future energy vector, both for use in light duty propulsion and as an enabler for other promising low-carbon power train innovations, particularly waste heat harvesting.

—Neville Jackson, Chief Technology and Innovation Officer, Ricardo plc



A.C. R.

According to this,


Centralized LN2 production requires about 400 Wh/kg. With LN2 providing 214 Wh/kg the efficiency is 54%. Unfortunately with an engine efficiency of 25% the electricity to traction efficiency is only 13%. That's very poor compared to battery/pluging hybrids 80%.

Decentralized production is 1.6 kWh/kg LN2, 4x the energy cost, so that's not going to happen.

What's the energy requirement of shipping LN2 in trucks? 214 Wh/kg compares very poorly to gasoline @ >12000 Wh/kg, at least 56x worse. So you'd have to ship more than 56x the tonnage of cryogen as you'd be shipping gasoline.

A.C. R.

Hmm, ok, a modern fuel tanker truck of 30000 liters guzzles some 30 liters of fuel per 100 km. So if the average distance from LN2 production plant to fuelling station is 100 km, that's a modest 0.1% of the fuel required for truck transport.

So for LN2 this would be pushed up to several percent of the charge. That's tolerable.

Having 56x as many fuelling trucks is going to cost money and cause a lot of logistics issues though...

Henry Gibson

Firefly batteries are advertized from an India company and should be cheap enough for refrigerated trailers in the near future for long life cheap lead battery powered free piston efficient compressors. LG makes such compressors for many years now for refrigerators.

ZEBRA and DURATHON batteries could be cheaper in the future, but Cell Phone service now takes most of the production.

It seems that both Tesla cars and Boeing planes should use these cells. Any savings from not using metal hydride batteries have been wiped out for the planes.



One thing that helps the economics of LN2 is that a lot of it is produced as a byproduct from the manufacture of liquid oxygen. Waste not, want not.


Another thought: CO2 is liquified on the way to producing LN2. So we could fight climate change twice using LN2, by #1 not burning oil in transport and #2 sequestering CO2 as a side effect of producing the fuel that replaces it.

A.C. R.

I can't imagine that excess nitrogen production is completely "wasted" right now.

Much more likely it is used to cool down incoming air in the process (regenerative/recuperative).


Well, everytime I read about Ricardo in the news, there will always be something fishy about the technology.

As per what ACR is saying, it takes a lot of energy to liquify nitrogen, huge!! Oxygen boils at -183C and nitrogen at -196C, you will not get liquified nitrogen yet even when oxygen is already liquified.

Transporting it around is also a big problem as you also need to keep it liquified.

A.C. R.

Good points bundy.

Nitrogen's lower boiling point indeed means that excess nitrogen production, in an energetic sense at least, does NOT exist.

That's interesting.

There is a "but" involved, though: ASUs currently produce neon/helium as the lowest boiling fraction, they are quite valuable gasses. If the demand for those gasses goes up (especially helium for helium cooled reactors if we can't get enough from terrestrial sources), the excess liquid nitrogen will be "free" at least energetically. Of course it is still a drop in the bucket on the global transport scheme, so at some point a lot of nitrogen will have to be produced purposfully to these LN2 engines, at which point the ASU energy consumption really hurts this scheme's economics.

A.C. R.

Hmm, thinking about that again though... if the LN2 has no market, it could be used to precool incoming air for the ASU to reduce neon/helium production energy consumption... so I guess even then, the LN2 is not "free" energetically... only as free as the inefficiency of the regenerator/recuperator process.


The article says there's available capacity, not that it wouldn't take energy to run it.

I'm somewhat mystified by the idea of using low-grade (cooling system) heat from a diesel engine, when there would be high-grade heat available from the same source (engine exhaust).  When you can more than double the absolute temperature of the gas, and thus its specific volume and recoverable energy, why wouldn't you?!


E-P, I'm just a layman so I'm only guessing, but could it have anything to do with the relative heat carrying capacity? The cooling system may be low-grade heat but it's working fluid is water which is dense so it has a high heat carrying capacity. The exhaust system OTOH may operate at higher temperature but hot gases aren't very dense.

I hope I'm using the right terminology.


The exhaust and radiator heat losses are roughly the same.

There are high-temperature heat-transfer fluids such as Dowtherm which could be used in an LN2-injection engine.  Heating the liquid up to 400°F (or higher) using engine exhaust would give much better output than using much cooler liquid from the cooling jacket.  If the goal is to squeeze out more energy from the fuel... why the suboptimal scheme?


Yes exhaust and radiator heat losses are roughly the same but what about the volume of the working fluids? Don't you have to pump more gases through the heat exchanger than you would if you used water? There HAS to be a reason they're using what seems to us a suboptimal scheme. Cost? Size of the heat exchanger? What?


You said "I can't imagine that excess nitrogen production is completely "wasted" right now."

The Dearman website says "The UK's industrial gas industry currently exhausts 8,500 tonnes of nitrogen each night. This would be enough for 6.5 million car kilometres or 42,500 buses daily."


There's far more volume of exhaust gas than engine coolant flow.  That doesn't make it hard to recover heat from it.

This may be just a failure of imagination, pure and simple.


LIN is cheap (was around $1/litre some time ago when I was involved), LOX is the valuable product. However the "vented Nitrogen" above is just that, vapour byproduct from LOX production. A.C. R. was correct.


Just because UK get rid of 8,500 tonnes of nitrogen every night, the nitrogen engine can use it right away. It is the cooling and transportation process that will mess up the entire energy balance.

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