The rebirth of the diesel engine

Automotive technology: Electric and hybrid cars are being given a run for their money by an unlikely competitor: a range of advanced diesel engines that set new standards in performance and fuel economy

Sep 7th 2013 |From the print edition

 

TESLA MOTORS has had great success with its Model S luxury electric car, which has outsold its petrol-powered equivalents since being launched in America last year. Even so, the prospects for battery-powered vehicles generally may never shine quite as bright again. Having had their day in the sun, they may soon be eclipsed by, wait for it, the diesel engine.

American readers will find this idea particularly hard to swallow. Surely not that dirty, noisy, smelly, lumbering lump of a motor that was hard to start in winter? Certainly not. A whole new generation of sprightly diesels—developed over the past few years—bear no resemblance to the clattering Oldsmobile 4.3-litre diesel of the late 1970s, which single-handedly destroyed diesel’s reputation in America for decades.

Later this year Americans will get their first chance to experience what a really advanced diesel is like—and why Europeans opt for diesels over hybrids, plug-in electrics and even petrol-powered cars. The leader of the new pack is the Mazda 6, with the choice of either a 2.5-litre four-cylinder petrol engine or a 2.2-litre turbo-charged diesel. The diesel has more than 30% better fuel economy and provides oodles more pulling power. Good as the petrol version is, motorists who choose it over the diesel will miss out on a lot. And Mazda is not the only carmaker with an advanced diesel in the works. Among others, Mitsubishi Motors has been selling cars with a new generation of 1.8-litre and 2.2-litre diesel engines in Europe since 2010. Hedging its bets on hybrids, Toyota has also been testing several radically new diesel designs.

What marks this latest generation of diesel engines from even their “common-rail” predecessors of the late 1990s, let alone their belching ancestors from the 1970s, is the use of a surprisingly low compression ratio of around 14:1 rather than the more usual 16:1 or higher. The reduction in cylinder pressure may sound marginal, but it gives rise to a virtuous cycle of beneficial effects that were previously unavailable.

For a start, the lower cylinder pressure reduces thermal and mechanical stresses in the engine. As a result, the heavy cast-iron block traditionally needed to stop a diesel ripping itself apart can be replaced with a lighter aluminium casting. That trims 25kg (55lb) off the weight of the block of the new Mazda diesel. Lower cylinder pressures mean that pistons, rings, valves, crankshaft and other engine parts can also be made 25% lighter. And because they are weighed down less by the engine, the vehicle’s brakes, suspension and bodywork do not need to be quite so rugged either. All these weight savings translate into greater efficiency. According to Ricardo, an engineering consultancy, every 10% reduction in a family car’s weight boosts its fuel economy by more than 4%.

Another benefit of lower cylinder pressure is that the lighter moving parts in the engine generate less internal friction—improving efficiency still further. And having less inertia, they allow the engine to spin faster and more freely, which also boosts efficiency. Mazda’s new “Skyactiv-D” engine can reach 5,200 revolutions per minute, a figure previously unheard of among road-going diesels. All told, the improvement in engine efficiency more than compensates for any loss of power caused by reducing the diesel’s compression ratio. As it is, diesels start off by being 30-35% more efficient than petrol engines. The new low-compression diesels are likely to be even more so.

There are benefits on the emissions side as well. In a typical diesel engine, ignition is caused not by a set of spark-plugs firing sequentially, but by the heat of the air being squeezed in the cylinders. The timing of this auto-ignition is controlled by the injectors, which squirt precise amounts of fuel under extremely high pressure into each cylinder exactly as needed. For maximum efficiency, this is done just as the pistons arrive at the top of their stroke and the cylinder pressure is at its highest.

Unfortunately, the fuel and air at top dead-centre are rarely mixed as thoroughly as necessary for complete combustion. This incomplete combustion produces soot particles and smog-forming nitrogen oxides—the curse of traditional diesel engines. Modern clean diesels trade some of their power for improved combustion. They do so by delaying the injection of the fuel until the piston begins to move back down the cylinder. The delay and the falling pressure give the fuel a chance to blend with the air better.

Even so, clean diesels still need an expensive catalytic-reduction system that injects a solution of urea into the exhaust to mop up the nitrogen oxides. They also need particulate traps to capture the soot. Going to a lower compression ratio avoids much of this. The fuel be burned without difficulty at the cylinder’s top dead-centre, and the urea-injection system is no longer required.

Not just a fair-weather friend

Meanwhile, the diesel’s old bugbear of poor starting in cold weather has been licked by the adoption of piezoelectric fuel injectors with multiple nozzles, which can spray fuel in whatever pattern best suits the operating conditions. And because the valves on modern engines have variable lift and timing, the exhaust valves can be left slightly open as the engine is coughing and spluttering during a particularly cold start. Hot exhaust gas sucked back into the cylinders then helps the engine warm up.

With its old 1.4-litre diesel engine, the Volkswagen Polo currently holds the record for being the most frugal non-electric car in Europe, with a fuel economy on the combined cycle of just 3.8 litres/100km (equivalent to 61.9 miles per American gallon). The Toyota Prius hybrid? A lowly 20th in the league table of the most economical fuel-sippers, with 4.2 litres/100km, along with higher emissions of carbon dioxide. The 19 cars having better fuel economy than the Prius hybrid are all clean diesels.

Your columnist fully expects the new generation of clean, low-compression diesels to improve fuel-economy by a further 20% or more. That will put diesels on much the same footing—given the way that equivalent miles-per-gallon are calculated for electric vehicles—as many battery-powered vehicles, but without any worries about range or recharging. Roll on the day.

From the print edition: Technology Quarterly

Grow Your Own Energy

A NASA-backed experiment harvests algae for oil, releases fresh water.

By Jonathan Trent|Posted Monday, Sept. 3, 2012, at 7:15 AM

Filamentous green algae.

All algae produce natural oils. We may just need to harness the right ones for an energy source.

Before we run out of fossil oil, we will thoroughly tap the sea floor, find and frack wells wherever they may be, and excavate and extract the most recalcitrant of oil shales. In so doing, we will fuel our lifestyle for a few more decades at the cost of releasing vast amounts of carbon dioxide, adding to global warming, melting ice caps, raising sea levels, acidifying oceans—and setting course for a future for which there are few optimistic scenarios.

In the face of all this, scientists are racing to find alternatives. Biofuels are my passion, but they have had rather a bad press, from complaints about displacing food production to the inefficiency of soybeans and the carbon footprint of ethanol. Microalgae have a low profile but they deserve a much higher one, since the fossil oil we mine mostly comes from microalgae that lived in shallow seas millions of years ago—and they may be key to developing sustainable alternative fuels.

Algae are single-celled organisms that thrive globally in aqueous environments and convert CO2 into carbohydrates, protein, and natural oils. For some species, as much as 70 percent of their dry weight is made up of natural oils. Through transesterification (the process of adding three molecules of alcohol to one molecule of natural oil), the algae oils can be transformed into renewable fuels.

Microalgae hold great promise because some species are among the fastest growing plants alive and are therefore one of the best sources of biomass, while other species have been estimated to produce between 18,700 and 46,750 liters of oil per hectare per year, nearly a hundred times more than soybeans' 468 liters per hectare per year.

But there are big unsolved problems at which governments should be throwing funds and brainpower as if we were involved in a Manhattan project. For example, since few species of microalgae have been domesticated, we don't know how to grow them reproducibly or economically. At what scale will algae farming be efficient? To put this in perspective, U.S. planes use 80 billion liters of fuel per year. To supply this fuel from microalgae at the lower end of the estimated production rate would take 4.2 million hectares—twice the area of Wales.

Luckily, there may be a good way to cultivate this much algae while solving the ethical problem of producing biofuel without competing with agriculture. Freshwater algae can be grown in wastewater (effectively, water with fertilizer), or marine algae can be grown in a blend of seawater and wastewater. In both cases, wastewater provides a growth medium and the algae clean the wastewater by removing nutrients and pollutants from it. So there's no competition for fresh water needed elsewhere, no reliance on synthetic fertilizer, and the environment benefits.

The United Nations estimates that the world produces around 1,500 cubic kilometers of wastewater annually, of which more than 80 percent is untreated. This means there is an ample supply of nutrient-rich water for the algae, while algae treatment is available to offset the environmental impact of wastewater.

There remains the question of how and where to grow the algae. A few species are cultivated commercially on a small scale, in shallow channels called raceways or in enclosures called photobioreactors (PBRs). Raceways are relatively inexpensive, but need flat land, have lower yields than PBRs and problems with contamination and water loss from evaporation. PBRs have no problems with contamination or evaporation, but algae need light, and where there is light, there is heat: A sealed PBR will cook, rather than grow, algae. And mixing, circulating, and cleaning problems send costs sky high.

Assuming we can fix this, the question of siting remains. In order not to compete with agriculture, PBRs must use nonarable land reasonably close to a wastewater treatment plant. But in most cities, wastewater plants are surrounded by infrastructure, so installing PBRs on thousands of hectares around the plants would affect roads, buildings, and bridges—again driving up costs prohibitively.

A solution occurred to me: For coastal cities, we should try a system I call OMEGA: Offshore Membrane Enclosures for Growing Algae. Some 40 to 60 percent of Earth's population lives near a coast, most of the biggest cities are near a coast, and nearly all coastal cities discharge wastewater offshore.

How does OMEGA work? It uses PBRs made from cheap, flexible plastic tubes floating offshore, and filled with wastewater, to grow freshwater, oil-producing algae. It would be easier to build the systems in protected bays, but breakwaters could also be constructed to control waves and strong currents. The water need not be deep or navigable, but a few things are crucial, including temperature, light, water clarity, frequency and severity of storms, boat traffic, nature and wildlife conservation.

Beyond solving the problem of proximity to wastewater plants, there are other advantages to being offshore. OMEGA uses buoyancy, which can be easily manipulated, to move the system up and down, influencing exposure to surface waves and adjusting light levels. And the overheating problem is eliminated by the heat capacity of the surrounding seawater.

The salt gradient between seawater and wastewater can also be exploited to drive forward osmosis. Using a semipermeable membrane, which allows water, but not salt, pollutants, or algae to pass through, wastewater is drawn into the saltwater with no added energy. In the process, algae are concentrated in preparation for harvesting and the wastewater is cleaned, first by the algae, and then by forward osmosis. This produces water clean enough to release into the marine environment or recover for reuse.

If OMEGA's freshwater algae are accidently released, they die in seawater, so no invasive species can escape into the ecosystem. In fact, OMEGA can improve conditions by providing a large surface for seaweed and invertebrates to colonize: part floating reef, part floating wetland. Then there are the extra possibilities of developing wind or wave power and aquaculture, growing food such as mussels.

OK, if it's so good, where is it? For the past two years, backed by NASA and the California Energy Commission, and about $11 million, we have crawled over every aspect of OMEGA. In Santa Cruz, Calif., we built and tested small-scale PBRs in seawater tanks. We studied OMEGA processing wastewater in San Francisco, and we investigated biofouling and the impact on marine life at the Moss Landing Marine Laboratories in Monterey Bay.

I'm now pretty confident we can deal with the biological, engineering, and environmental issues. So will it fly economically? Of the options we tested, the OMEGA system combined with renewable energy sources—wind, solar, and wave technologies—and aquaculture looks most promising. Now with funds running out and NASA keen to spin off OMEGA, we need the right half-hectare site for a scaled-up demonstration. While there is enthusiasm and great potential sites in places ranging from Saudi Arabia to New Zealand, Australia to Norway, Guantanamo Bay to South Korea, as yet no one has committed to the first ocean deployment.

We could be on the threshold of a crucial transition in human history—from hunting and gathering our energy to growing it sustainably. But that means getting serious about every option, from alpha to OMEGA.

This article originally appeared in New Scientist.

*$2.28 per gallon algae biofuels?

In California, OriginOil announced a new company study indicating a potential production cost as low as $2.28/gallon ($0.60/Liter) for gasoline or diesel using a blend of algae and waste feedstocks, using the latest growth, harvesting and fuel conversion technologies from OriginOil and other innovators. OriginOil’s comprehensive model analyzes the entire algae production process at scale, integrating the latest advances in growth, harvesting and fuel conversion.

In the lowest-cost scenario, algae harvested using OriginOil’s Algae Appliance is blended with waste feedstocks and converted onsite to achieve a modeled production cost of $2.28 per gallon for gasoline or diesel. This cost roughly doubles to $5.44/gallon ($1.44/Liter) when using pure algae feedstocks. The model assumes a production footprint of at least 50 hectares (124 acres). Source 'Biofuel Digest'