New way to get that vital spark

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The automotive industry is under ever-increasing pressure to increase fuel efficiency and reduce levels of harmful emissions. In principle, laser ignition offers the potential to deliver both; in practice, the combination of foreseeable problems and uncertainties involved in using a laser to power combustion in mass produced petrol engines has deterred most researchers. Recently, however, engineers at The University of Liverpool reported encouraging results, following a collaboration with Ford Motor Company and a laser manufacturer. 

During the second world war the Germans and the Soviets increased the fuel efficiency of their aircraft engines by injecting aviation fuel directly into the combustion chamber of each cylinder. Fifty years passed before the same technique was made to work well in the engines of mass produced cars, but the range of vehicles fitted with ‘gasoline direct injection’ (GDI) engines has been growing steadily since the turn of the Millennium. 

The 1955 Mercedes Benz 300SL ‘Gullwing’ was the first gasoline-powered car equipped with fuel injection directly into the combustion chamber. No other mass-produced car featured this until the mid-1990s when Mitsubishi Motors introduced GDI models like the Carisma.

This is good news from the perspective of dwindling oil reserves, but GDI won’t realise its full potential until there’s an alternative to the 90-year old technology we still rely on to ignite the fuel. Spark plugs impose severe constraints on our ability to achieve ultra lean burn by manipulating combustion: they ignite the charge in a fixed position, so they can’t cope with a stratified charge. 

Igniting the charge using optical energy generated by lasers could circumvent spark plugs’ limitations but when the idea emerged, lasers were too big to fit in an engine compartment and our understanding of the technology was still in its infancy. Since then, we’ve learned how to harness lasers in myriad applications and make them a lot smaller.  

The University of Liverpool has the UK’s oldest and largest academic research group specialising in lasers. For 20 years its Laser Group has advanced our fundamental understanding of the technology and helped industry to get the best from it. In 2002, members of the group joined forces with colleagues from the Powertrain Laboratory and took on a new challenge: developing a laser ignition (LI) system which could be used with existing GDI engine designs.

Engineers in the US and Europe had already demonstrated successful LI in gas turbines and rockets, but automotive engines were a very different proposition and previous research in this area was limited to preliminary studies on basic combustion processes. “That’s not surprising”, comments the head of the Powertrain Laboratory, Dr Tom Shenton, “since it was obvious it would be a risky enterprise.” 

A successful outcome could bring significant benefits, however: a LI system should reduce not only fuel consumption, but also emissions.  

With increasingly stringent limits on emissions from new vehicles due to come into force in the EU over the decade, the UK’s Foresight Vehicle Link Programme invited researchers to submit proposals for projects which would reduce gaseous emissions significantly and help achieve reductions of 50 per cent or more in hydrocarbons, carbon monoxide and nitrous oxide emissions.  

Tom Shenton had a long-standing research relationship with Ford Motor Company, while his colleague, Geoff Dearden of the Laser Group, had already collaborated with Spectron Laser Systems (now part of the GSI Group). They proposed – in partnership with Ford and Spectron – a study to determine the feasibility of LI systems for automotive engines fuelled by gasoline (petrol). The bid was successful, attracting £660,000 in public funding, with additional, in-kind funding from Ford and Spectron. 

In principle, it is possible to achieve LI thermally, photochemically, or through resonant or non-resonant break-down. The team opted for non-resonant breakdown, as it is the most similar to electric spark ignition. “Experience suggested that a laser operating in the near-infra red spectrum would be appropriate for this project – and that proved to be the case”, says Dr Dearden.  

They considered a number of different ways to deliver the laser beam to each of the four cylinders in the test engine. “You could have an individual laser for each cylinder”, Geoff Dearden explains. “We felt that was unlikely to offer the best solution in the long run, but it made for a good starting point experimentally. You could use a single laser, or a pair of laser heads, and split the beam – or deliver the beam via an optical fibre. We tested all three methods.” 

A laser beam angled down through the optical plug below the aperture and into the research engine’s single cylinder

Since their main aims were to prove the principle of LI, explore the benefits and optimise the parameters, they created an optical bench beside the test engine, with the laser fixed to it horizontally – although they also ran some experiments with the laser positioned above the test engine. They used two beam splitters and four engine mirrors mounted at a 45° angle to direct the beams towards the cylinders. Initially they channelled the beam into the combustion chamber through an access hole formed by removing a sensor from a conventional spark plug. Later, they created an optical plug by fitting an optical lens at one end of a hollow spark plug and an optical window at the other. They also constructed a series of plugs with different focal points in order to establish whether their laser ignition system could cope with a stratified charge.  

The team was particularly keen to deliver the beam via optical fibre, since this was likely to be less susceptible to engine vibration and could facilitate improved engine layout. They tried out a range of optical fibres, including silica and sapphire, and experimented with different internal fibre structures, core sizes and beam coupling optics. 

They tested the effects of different laser energy levels; different beam profiles; different pulse durations; and different depths of focus to control the distribution (in time and space) of optical energy within the combustion chamber.  

Test results

Above: Firing a laser beam through the optical spark plug produces a bigger and more intense spark (top) than a conventional spark plug connected to a high-tension lead (bottom).

Above: This can be seen much more clearly when photographed in the dark.

Watch a video watch a video of the two plugs sparking (you will need RealPlayer to be installed on your computer).

The team achieved successful LI in individual cylinders of their test engine early in the research programme; this was followed by LI in pairs of cylinders; then they started the engine using LI alone. By the end of the project they were able to run the test engine with combustion in all four cylinders powered by the laser, although at this stage they didn’t run it continuously for hours. “With the right laser settings, LI can outperform conventional spark plug ignition, since it allows the engine to be run on leaner fuel mixtures”, Tom Shenton reports. 

Delivering the beam through free space and channelling it into the combustion chamber through the optical plug achieved the best results – reducing the Coefficient of Variation, making combustion smoother and more fuel-efficient. The team identified the optimum LI configuration to achieve the best combustion stability and showed that LI enabled a greater operational window of engine parameters to be exploited, leading to reduced exhaust emissions over the full range of engine load conditions. 

Delivering the beam via optical fibre proved to be more difficult than the team had hoped. “The fibre didn’t respond well to engine vibration, which increased the divergence of the output beam and reduced the beam mode quality”, Geoff Dearden reports. “Bending the fibre was also problematical: up to 20 per cent of the beam energy was lost with small bend diameters, while tight bends caused the fibre to fail altogether after a period. What’s more, the high density of laser energy can cause short or longer term degradation, causing loss of beam transmission – and therefore ignition. Careful design of laser parameters, fibre coupling and choice of optical media is crucial to avoid this. We’re confident that we can solve these problems with further research.” 

The project generated a series of invention disclosures, some of which have already been submitted for patent protection. 

This month the team – expanded to include Paul Dickinson and Jack Mullett – is embarking on the next phase of the research, funded by the Carbon Trust. Once again, they are working in partnership with Ford. Their aim is to develop LI systems for next-generation car engines based on efficient, downsized gasoline direct injection (GDI) technologies. In the process, they expect to increase engine efficiency and further reduce exhaust emissions.