Dolphin N2 and The Recuperated Split Cycle Engine

Dolphin N2 is a subsidiary of FPT Industrial, the powertrain division of CNH Industrial. Acquired by FPT in 2019, Dolphin N2 was originally created by Ricardo PLC to develop the Recuperated Split Cycle engine. The name “Dolphin” is taken from the first engine developed and produced by Sir Harry Ricardo, also a type of split cycle, the popularity of which laid the foundations of Ricardo PLC.

The Recuperated Split Cycle engine has the potential to reduce fossil fuel use by up to 30% whilst simultaneously reducing air pollutants to well below any future planned legislation. It is also demonstrated to run on both hydrogen and biomethane. The concept is covered by 8 international patent families. Since 2017, a single cylinder proof of concept engine has been running at Brighton University; the first multi-cylinder prototype is now in development.

What do ThermoPower® and CryoPower® do? 

The Recuperated Split Cycle is an internal combustion engine, with all that implies in terms of low cost and ease of manufacture, that aims to compete with zero emission drivetrains. It targets long haul trucks (where zero-emission technology is least easy to apply), 0.5-50MW distributed power generation (an area seeing rapid growth to reinforce electricity grids and balance out intermittent renewables), mixed-mode rail, and marine applications. It can potentially do this with the efficiency of the largest power stations, and air quality somewhere between the toughest Californian SULEV standard, and zero-impact (meaning emissions can’t be detected or are cleaner than the surrounding air).

There are in fact two versions of the technology: ThermoPower® is a simplified system offering most of the air quality benefits and efficiency advantage versus advanced diesel engines in research; CryoPower® delivers ultimate efficiency and air quality by addition of Liquid Nitrogen or Air to its internal processes.

How do ThermoPower® and CryoPower® work?

ThermoPower® and CryoPower® are Recuperated Split Cycle Engines. The revolutionary step is to separate the “cold” and “hot” parts of the traditional internal combustion engine.

A first set of cylinders draw in air and compress it – in the CryoPower® version, Liquid Nitrogen is injected to keep this process cool for maximum efficiency; the simpler ThermoPower® injects water. Then the compressed air passes through a recuperator, where the engine’s exhaust heats it up – saving fuel which normally has to do this. The air now passes to the second, hot cylinder set, which are thoroughly insulated – infeasible in a normal ICE where the same cylinder handles hot and cold processes. These cylinders host the combustion and expansion events, which produce power; they are bigger than the compressor cylinders (again impossible in a standard engine) because that is most efficient.

As a final twist, the hot air passing into these combustion cylinders does so at the speed of sound, leading to extraordinary mixing with the fuel, and a unique “cool combustion” regime. The very low level of emissions, especially NOx and Particulates, can be reduced to at least SULEV and potentially lower using known “SCR” (Urea-based) after-treatment.

Is it futureproof? 

It is tempting to see any ICE as a “transition technology”. However, combustible fuels remain by far the most energy-dense in comparison to batteries; they can be manufactured sustainably from bio-mass or synthesised.

The split-cycle combustion system has been demonstrated on Diesel, Natural Gas and hydrogen; and it is potentially compatible with napthas, ethanol, hythane and ammonia.

The Liquid Nitrogen is already sustainable, as air-separation is a major player in electricity “demand-side management”; as use of renewables rise, it has been shown to be an attractive way of balancing the grid.

How well validated is the technology? 

Split-cycle engines are known technology, and recuperation is commonplace in industrial gas turbines. It is the specific combination of these, plus the use of water or liquid nitrogen, that is innovative. Ricardo’s work on recuperated split cycle engines started in 1992 in a project with the UK energy utility National Power, leading to two demonstrator units.

The ThermoPower® / CryoPower® system itself was first validated using a single cylinder Titan engine with recuperator (the “hot half”), to study low emissions behaviour; since the end of 2020, a multi-cylinder “mule” prototype has been running to validate the whole cycle and develop durable solutions to key systems.

Specific rig tests are being used to study critical aspects such as water or liquid nitrogen injection and the fast-acting intake valves. Use of liquid nitrogen in vehicle and mobile applications is well known, with four companies offering it for truck refrigeration; refuelling is straightforward and similar to Liquified Natural Gas; the simpler water injection option is based on well-known SCR technology.

What is the business case? 

The business case for the technology is based on much lower CapEx than low emission alternatives, and a rapid payback versus conventional (usually Diesel) technology.

On-cost for the ThermoPower® engine is estimated at around +20% vs conventional Diesel, with a fuel saving of 10-18% giving payback in a year or less; for CryoPower®, capital cost is estimated at around 50% of the cost of a commercial Diesel engine; fuel cost savings of up to 20-25% per year will again give a rapid payback on the extra cost.

What are the plans for development? 

In Phase 1, we are developing the multi-cylinder prototype engine to a full demonstrator, with the support of parent FPT Industrial and two grant-funded consortium projects, StepCO2 (InnovateUK) and RE-ARMD (APC).

This will be followed by a more integrated, Phase 2 engine that will be demonstrated in a truck. From there, we anticipate industrialisation of the technology for sale in multiple markets / applications in the second half of the decade.