Renault probes two-cylinder two-stroke diesel

Some thermodynamic cycles attract powertrain engineers as habitually, they revisit the past in a bid to create the future.

None more so is this true than with the two-stroke engine. Ford’s brush with the Orbital Engine Corporation’s two-stroke gasoline engine in the 1980s and tested at the Millbrook Proving Ground, Bedfordshire, is a reminder of what might have been.

And observers are still waiting patiently to see the tangible results of the tie-up between China-based Zhongding Power and EcoMotors, the US company that claims (notwithstanding the former efforts of Commer, Junkers and Napier, and more recently Achates Power Inc.) it has developed an opposed-piston (opoc) two-stroke engine.

EcoMotors claims its diesel engine has the potential to be the world’s cleanest, most efficient, lightweight and lowest-cost internal combustion engine, with the potential for 20-50 per cent better fuel economy compared to a state-of-the-art diesel engine.  

Now it seems Renault has been studying a two-diesel also, but not on the scale of the EcoMotors’ design which could be used for commercial vehicles and power generation. And Renault’s design is not an opposed piston configuration.

Instead, Renault is looking at a two-cylinder, 1-litre two-stroke diesel engine which is harnessing turbocharging and supercharging, both of which add cost, weight and complexity but are necessary to get air charge into the cylinder quickly. In theiry, this new two-cylinder two-stroke could be doubled up to make a 2-litre two-stroke four-cylinder engine - IF the full two-stroke dynamics of emissions, NVH and fuel economy can be satisfied.

The problem with the two-stroke is that everything has to happen within 360 degrees of crankshaft rotation. A four-stroke has 720 degrees, so much more time is available to handle induction, injection, combustion, power delivery and exhaust. Even then there is not enough time to solve the emissions issues completely.

Renault engineers claim they can achieve cost savings of 30 per cent while still meeting Euro 6 emission requirements. Two very important parameters.

The main advantage of the two-stroke is one of cost – fewer components and therefore theoretically much simpler items.

Diesel engines are more expensive to produce and a diesel that has fewer moving parts should be cheaper. In previous years, Detroit Diesel in the US (its two-stroke engines were available in over 200 truck models), Foden in the UK, and Krupp in Germany developed and manufactured two-stroke diesel engines.

Renault engineers have already been testing an 0.73 litre two-stroke diesel fitted with an Eaton supercharger and a turbocharger from CV Turbo of the Czech Republic. These two items alone add cost to what is basically a simple engine. So the dynamics of a low-cost engine are already changing.

And it is the total cost of the package that determines is suitability and acceptability as a future powertrain, possibly to be linked into a hybrid role.

But by upping the cylinder volume to 1 litre engineers also can widen the scope of vehicle application to include very light vans as well as passenger cars.

From a manufacturing perspective, Renault needs production volume to hold down cost, and volume implies spreading its use to a wider range of vehicle models, hence 1-litre.

According to one Renault engineer, as reported in Engine Technology, “We have technical solutions to limit exhaust pollutants, but it is the total cost of the whole system to limit and treat the pollutants that is the problem.”

“We think the two-stroke is one way to keep the cost down and yet have the correct efficiency and emissions level,” he added.

Renault has shown in the past that it is no slouch when it comes down to diesel technology. So its engineers have to be given a hearing.

In their prototype, Renault engineers cut a 1.5-litre, four-cylinder engine in half and used the same bore and stroke, as well as much of the bottom end of the engine. The main changes came with the cylinder head, combustion system and bottom-end tribology.

And, by teaming up with Delphi – formerly Lucas Diesel Systems with facilities in the UK and France – engineers claim they can produce 145Nm of torque from 1,500rev/min as well as 70PS power output.

Dyno testing and real-world on-the-road evaluation in a Renault Twingo has pointed the way to a strong case for further evaluation, with NVH as one of the subjects to study further now that the principles have been proven.

Certainly, engineers have learnt enough from the prototype for someone in an important position, and with a handle on cost-benefit, to consider which way to go in taking the next step.

                                       US development

Meanwhile, in the US, engineers at Achates Power Inc. of San Diego, California, continue with their development of an opposed-piston two-stroke diesel engine. But reports of applications are still awaited.

Achates Power claims it is actively engaged with “nearly all of the leading commercial truck and passenger vehicle manufacturers worldwide”. Quite a bold statement.

It adds: “Through our contracts with the U.S. Army and Fairbanks Morse Engine, we’ve also expanded into the military, maritime and stationary power industries”.

The engineers and scientists who started Achates Power Inc. in 2004 did so “with the audacious idea that innovation and modern technology could transform the proven and record-setting two-stroke opposed-piston engines of the past into the clean and efficient engines of the future”.

Taking advantage of advances in analytical tools and the plummeting cost of computing, Achates Power claims to have solved the fundamental problems which had previously hindered the development of two-stroke opposed-piston engines, thus “unleashing their full potential”.

Achates Power says it earns revenue in two ways: By offering engineering services to customers, designing or helping to design engines based on our proprietary technology. “Our customers then manufacture these engines for use in their own applications”, it claims.

Achates Power also offers a technology license for access to designs, expertise, tools, control software and patents. With this technology license, we earn a royalty for each engine sold by our customers.

The company claims to have extensive prototyping capabilities and state-of-the-art test facilities are instrumental to confirm at every step the validity of its analytical approach and results. Nevertheless, the company is no doubt following developments at Zongding/EcoMotors very carefully to see if they have any technology it can use also.

Achates Power claims its proprietary cylinder and piston designs achieve “unprecedented improvements in combustion efficiency and oil consumption to meet the most stringent emissions regulation. In conjunction with the thermal efficiency advantage inherent to opposed-piston engines, our designs realize significant reductions in fuel consumption over conventional four-stroke compression ignition engines. Based on its unique technology, Achates Power is creating clean, more fuel efficient and lower cost engines for the 21st Century".

Ford’s relationship with Orbital did not last very long; the work centred mainly on Ford’s Research and Engineering Centre at Dunton, Essex, UK. Maybe by going it alone Renault will prove that two-strokes do have a positive role to play, especially in a hybrid.

However, the Zongding/EcoMotors opposed-piston engine could be the first two-stroke to roll out on 2015. And, when the announcement does come, it is likely to include an important, ground-breaking engineering statement. 

• On a separate but political front, Cherie Blair, wife of former UK prime minister Tony Blair, is in line to take a seat on the board of directors at car maker Renault. Mrs Blair, 60, a human rights barrister, is set to be proposed as director for four years at the French company’s annual shareholder meeting at the end of April. The move comes as Renault, the world’s fourth largest car maker by sales through its alliance with Japan’s Nissan, is looking to increase the number of women on its board. It currently has 19 members, of whom just four are female. Renault is reported as noting that Mrs Blair had been approached because, as a British woman, she would improve the “balance” and diversity on its board. She would also add useful experience in human rights and employment law. The move from the courts to the boardroom would be the first appointment to a French corporation for Mrs Blair. She is already on the UK boards of several charities.