As anyone who has accidentally put petrol into their diesel vehicle knows, it’s usually an experience to be avoided at all costs. Yet at a workbench littered with coloured wiring, pipes and sensor displays, Steve Ciatti is doing precisely that. What is an expensive blunder for most drivers is an opportunity for Ciatti to combine the efficiency of diesel with the relative cleanliness of petrol.
For fans of the Nissan Leaf, the Tesla Model S, and the Vauxhall or Opel Ampera (the Chevrolet Volt in the US), the future of the car is electric, and the days of the internal combustion engine are numbered. But it’s far from a universal view, and for now at least engines that burn gases will continue to dominate car sales for years to come. So engineers continue to work to squeeze every last mile per gallon from them.
At the US Department of Energy’s Argonne National Laboratory, outside Chicago, Illinois, this means taking engines to new extremes, sometimes in vehicles, sometimes isolated in special test cells. Hence Ciatti’s outlandish sounding experiments.
“Literally, don’t try this at home!” he says. “It certainly is not a natural thought to run gasoline in a diesel engine, but when you control it properly, then not only can it be done, but you can achieve very impressive efficiency numbers, across the board. The emissions signature is significantly lower.”
Both petrol and diesel engines use a controlled explosion of fuel inside a cylinder to drive a piston. The pistons from the (usually four or six) cylinders are connected to a crankshaft, which creates circular motion that in turn drives the wheels, via the gearbox.
The way that the two types of engines trigger that explosion is very different though. A petrol engine has spark plugs, which can be fired to ignite the fuel and air mixture. Diesel engines, on the other hand, do not have spark plugs. The fuel ignites of its own accord when it is injected into compressed (and therefore heated) air in the engine – a process known as compression ignition.
Diesel engines give drivers more miles to the gallon. Part of the reason is that the power level in a petrol engine is determined by a throttle, which performs its role by controlling incoming air flow. Less air means less oxygen to fuel the explosion in the cylinder. However the engine has to work harder to suck in the air, resulting in a loss in efficiency.
That said, diesel engines have their limitations, mostly because burning diesel creates more soot and nitrogen oxide (NOx). The problem is that diesel actually burns too easily. It starts combusting as soon as it is injected into the hot air in the cylinder, before it has had a chance to mix properly with the air. The chemistry is complex, but the result is an inefficient combustion process that generates soot, or particulate emissions, around the stream of burning fuel in a similar way to a flickering candle.
Ciatti believes it is possible to keep the efficiency gains while dropping emissions significantly. “I can add gasoline much earlier than I could possibly add diesel, getting it better mixed, and helping to cut my soot numbers down,” he says. Tests at Argonne have shown that by putting petrol in a diesel engine can cut NOx emissions by 90% and soot by half, according to Ciatt. He is focusing on bringing soot levels down even further.
Because the gasses in the cylinder have more time to mix, Ciatti can throw another emission-cutting strategy into the mix. Exhaust gas recirculation (EGR) involves pumping some of the engine’s exhaust fumes back into the cylinder. This reduces the oxygen levels, and so cuts NOx emissions. The cost of this reduction in pollution is a loss of efficiency.
“I can bring the ambient air oxygen concentration down by diluting it with exhaust gas,” says Ciatti. “Because I am injecting it early, and don’t have a mixing problem like diesel does, I can run EGR without much penalty and I can bring my NOx levels to very low levels. I’m simultaneously able to reduce particulates and NOx because I am running a fuel that is difficult to auto-ignite.”
For now the experiments are restricted only to the lab. “If I tried to do this under the hood of a vehicle, it would be very difficult to do because I don’t have the ability to instrument or control it the way I do here,” says Ciatti.
However, the team at Argonne is working with major automakers, and oil companies, in the hope of changing that. The humble internal combustion engine, which has been powering cars since the 19th Century, may have some life left in it yet.