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Diesel may not be king for much longer in the hard-working 4WD arena.

It’s accepted wisdom that there’s no alternative to the diesel for heavy duty 4WD tasks. Two highly-qualified US researchers disagree and have come up with a petrol-fuelled alternative. The editor sourced this MIT Energy Initiative report.

As diesel engine makers struggle to meet increasingly demanding emissions laws, vehicle buyers are looking for lower-cost, less maintenance-intensive alternatives. Could the petrol engine make a return to heavy-duty 4WDing?

Several years ago, MIT’s Daniel Cohn and Leslie Bromberg, principal research engineer at the Plasma Science and Fusion Center and the Sloan Automotive Laboratory, took on the challenge of designing a low-emission, fuel-efficient replacement for the diesel engines traditionally viewed as the only viable powerplants for heavy-duty trucks.

This research was supported by the Arthur Samberg Energy Innovation Fund of the MIT Energy Initiative and the researchers were photographed by Stuart Darsch, of Boston.

Diesels require increasingly more complex emissions control systems that are now second only to tyre maintenance in US fleet operating costs.

Using sophisticated computer models developed by Bromberg, the two scientists have produced a conceptual design for an engine that should be up to the task.

Most efforts to reduce the adverse air pollution and climate impacts of today’s vehicles focus on cars and light-duty trucks, with strategies that range from electrification and carpooling to autonomous vehicles.

“These strategies can be an important part of the overall solution,” said Daniel Cohn.

“But it’s also increasingly important to think about heavy- and medium-duty trucks.

“Finding a way to clean them up could bring a great improvement in worldwide air quality during the next few decades.”

Powered largely by diesel engines, those trucks are now the largest producer of nitrogen oxide (NOx) emissions in the transportation sector, contributing to ground-level ozone, respiratory problems and premature deaths in urban areas.

Some estimates project that diesel fuel — used for both trucks and cars —will out-sell gasoline worldwide within the next decade, threatening to further increase already-severe urban air pollution as well as greenhouse gas (GHG) concentrations.

Today’s heavy-duty diesel engines provide fuel efficiency and high power and torque, making them ideal for long-haul, high-mileage commercial vehicles, but finding another option is critical, said Cohn.

“We need to replace diesel engines with other internal combustion engines that are much cleaner and produce less greenhouse gas.”

Using computer simulation analysis, Cohn and Bromberg have designed a replacement, half-sized gasoline-alcohol engine that should be not only cleaner, but also much lighter, lower-cost and higher-performing.

Within the USA, pressure on the trucking industry to deal with diesel emissions has been mounting. Expected regulations in California would require that NOx emissions from medium- and heavy-duty trucks be cut by about 90 percent relative to today’s cleanest diesels, which use complex and expensive exhaust
treatment systems just to meet current regulations.

In some parts of the world, notably India and China, those clean-up systems aren’t widely used. As a result, NOx emissions are about 10 times higher.

In the USA, some trucks have met strict NOx limits using large spark-ignition (SI) engines fuelled by natural gas. But large-scale adoption of those engines is problematic. Storing and distributing a gaseous fuel raises vehicle cost and poses infrastructure challenges, and the use of natural gas can lead to a heightened climate impact because of the leakage of methane, a GHG with high global warming potential.

To avoid the challenges of dealing with natural gas, Cohn and Bromberg decided to pursue another approach: a heavy-duty SI engine fuelled by petrol.

In general, petrol SI engines produce low NOx emissions. Guided by their computer models, Cohn and Bromberg took a series of steps to increase the power and efficiency of that design without sacrificing its emissions benefits.

During normal gasoline SI engine operation, the process of translating the combustion of gases into torque at the wheels progresses smoothly, until there’s a need for high-torque operation, for example, to pull a heavy load at high speed or up a hill.

Under this increased load, pressures and temperatures inside the cylinders can rise so much that the unburned combustion gases spontaneously ignite. The result is pre-ignition or ‘knock’ that’s characterised by a loud, metallic clanging noise and can destroy the engine.

The need to prevent knock has limited improvements in efficiency and performance that are needed for petrol engines to compete with diesels. However, we’re now seeing direct injection, turbocharged, 1.2-2.0-litre petrol engines in passenger cars delivering diesel-like economy and performance, and these
engines have virtually eliminated the sub-two-litre diesel class in China, Europe and the USA.

For a medium and heavy truck petrol engine the task is more difficult, but Cohn and Bromberg dealt with that problem using alcohol. When the SI engine is working hard and knock is imminent, a small amount of ethanol or methanol is injected into the hot combustion chamber, where it quickly vaporizes, cooling the fuel and air and making spontaneous combustion much less likely.

In addition, because of alcohol’s chemical composition, its inherent knock resistance is higher than that of petrol.

The alcohol can be stored in a small, separate fuel tank, just as AdBlue exhaust-clean-up fluid is stored in a diesel engine vehicle. Alternatively, it can be provided by on-board separation of alcohol from gasoline in the regular fuel tank. (As in Australia, most petrol sold in the United States is now a mix of 90 percent petrol and 10 percent ethanol.)

With concern about knock removed, the researchers took full advantage of two techniques used in today’s passenger cars: firstly, they used turbocharging, but at higher levels and, secondly, they used a high compression ratio.

The researchers also made use of an important feature of the low-NOx heavy-duty SI engine fuelled by natural gas: a mixture of just enough air to burn all the fuel — no more, no less.

That ‘stoichiometric’ ratio permitted important changes not possible in the diesel cycle that operates with excess air.

With stoichiometric operation, a three-way catalyst is sufficient to clean up the engine exhaust. A relatively inexpensive system, a three-way catalyst removes NOx, carbon monoxide, and unburned hydrocarbons from engine exhaust and is key to the low NOx achieved in today’s SI engines.

Then, given stoichiometric operation combined with a higher level of turbocharging and a high compression ratio, the researchers were able to shrink the engine size. The SI engine can’t use the excess air that’s in a diesel, so the total volume of its cylinders can be smaller.

“Because of that difference, you can replace a diesel engine with an SI engine about half as big,” said Bromberg.

With that reduction in size comes an increase in fuel efficiency. In any engine, the process of pumping air into the cylinders and various sources of friction inevitably reduce fuel efficiency. Those pumping losses depend on engine size. Make an engine smaller, and there’s less friction and less wasted fuel.

Taken together, the low-cost three-way catalyst and smaller overall size help make the gasoline-alcohol engine less expensive than the cleanest diesel engine with a state-of-the-art exhaust-clean-up system. Indeed, according to the researchers’ estimates, the cost of the gasoline-alcohol engine, including its exhaust-treatment system, would be roughly half that of the cleanest diesel engine.

How does the proposed half-sized petrol-alcohol SI engine compare with today’s cleanest full-sized diesel on efficiency and power? To answer that question, the researchers used a series of sophisticated engine and vehicle simulations and chemical kinetic models developed by Bromberg.

For the comparison, they used an illustrative version of their engine, based on a Cummins B6.7 engine that could — with relatively small alterations — be converted to the petrol-alcohol configuration.

The MIT analysis assumed that the 6.7-litre petrol-alcohol SI engine’s compression ratio was about the same as in a 12-litre diesel engine.

So, where could power and torque increase come from? The answer is that the SI engine can run far faster than the diesel can, because combustion is faster with spark ignition than with the compression ignition used in diesel engines.

Because of faster operation the small engine can produce almost 50 percent more power than the diesel can. The big bore petrol engines used in North American pickups, running low-octane petrol can easily produce around 500hp.

While the gasoline-alcohol engine is somewhat more efficient than the diesel at high torque and less efficient at low torque, in general the small SI engine is about as efficient as the diesel.

The accompanying graph shows efficiency, ethanol proportion and peak torque – 2000Nm isn’t bad from a 6.7-litre SI engine is it?

However, as more torque is required, knock becomes more likely, so more ethanol is needed. At the highest torque, about 80 percent of the total fuel must be ethanol to prevent knock.

That estimate raises a concern: in the United States, as in Australia, ethanol is widely used in a low-concentration mixture with gasoline, but pure ethanol or a high-concentration ethanol-gasoline blend may not be available or may be too costly. So how much ethanol is likely to be required for a given trip?

As an example, the researchers considered a trip taken by a long-haul, heavy-duty vehicle that requires high torque most of the time. Depending on the compression ratio, ethanol could make up 20 to 40 percent of its total fuel consumption. In contrast, a delivery truck might operate at low torque most of the time and do just fine with ethanol as 10 percent of its total fuel over a driving period.

“Such levels of ethanol consumption are do-able,” notes Cohn.

“But the system would be more attractive to people if you could use less ethanol.”

One way to reduce ethanol use would be to dilute the ethanol with water. Using the knock model, Cohn and Bromberg determined that knock resistance is actually higher when water makes up as much as a third of the secondary fuel.

“And in some cases where you don’t need any ethanol for antifreeze, you might be able to run with water alone as the secondary fluid,” said Cohn.

Another approach to reducing alcohol use is called up-speeding, or operating the engine at a higher speed. Running the engine faster and adjusting the gearing in the transmission to increase the ratio of engine rpm to wheel rpm make it possible to use less engine torque in the petrol engine to achieve the same torque at the wheels as in the diesel.

According to the researchers’ calculations, that reduction in engine torque could reduce ethanol use over a driving period to less than 10 percent of the total fuel consumed, an amount that could be supplied by on-board fuel separation.

Cohn pointed out one more benefit of the petrol-alcohol SI engine: a pathway to reducing GHG emissions.

“A somewhat under-recognised aspect in evaluating the environmental impacts of transportation vehicles is that GHG emissions from trucks worldwide will overtake GHG emissions from cars sometime between 2020 and 2030,” he noted.

The petrol-alcohol SI engine can be operated in a flexible-fuel mode where it uses only pure alcohol if desired. Looking at the life cycle of the fuels and assuming comparable engine efficiency, using ethanol produced from corn by state-of-the-art methods generates about 20 percent lower GHG emissions than using petrol or diesel fuel.

Even greater reductions in GHG emissions could come when ethanol and methanol fuels are produced from agricultural, forestry, and municipal waste or specialty biomass, as has been happening for years in Brazil, where alcohol is the primary automotive fuel (see below).

“Reducing GHG emissions from trucks by finding an alternative source of power, such as electrification could take a long time,” said Cohn.

“But if you can operate your engine partially with ethanol or entirely with ethanol, that’s a good way to make a start right away.”


Cummins fuel initiatives

The MIT researchers are no pie-in-the-sky academics. Their SI engine R&D is mirrored by Cummins’ own efforts.

I asked Cummins if that company was involved with the MIT research project, because it’s well known that Cummins, through its Westport subsidiary, has been producing SI engines, mainly fuelled by natural gas, for years. Cummins declined to comment on the MIT initiative, other than to say that it continues to investigate advanced petrol engine technology.

In late 2017, Brett Merritt, executive director for on-highway business at Cummins, stressed the company’s technical expertise and highlighted its wide product portfolio, noting the company is investing $700 million annually around the world in research and development.

Although Cummins believes diesel will remain the power source of choice for North American trucking for decades to come, the company is continuing to develop and invest in alternative fuel engines and technologies.

Cummins has been working on high-efficiency spark-ignited technology “that can deliver diesel-like performance and durability” across a range of liquid fuels from ethanol to methanol and petrol and can meet “the most stringent emissions requirements”.

At its Columbus, Indiana, technical facility, Cummins operates 88 test cells running diesel, natural gas, petrol, ethanol, hydrogen, propane and biodiesel engines and has also invested in and continues to explore fuel cell power.


Brazil’s alcohol dependency

Ethanol, produced in Brazil from sugar can rather than corn, as in the US, has been used in Brazil for many years.

The first ethanol-only powered cars went onto Brazilian roads late in the 1970s, after the first Oil Shock caused a serious foreign currency crisis in Brazil.

A dispute between sugar cane farmers and the Government sent ethanol into decline in the 1990s, but after that resolution ethanol-only cars made a comeback.

However, in 2003, the availability of electronic fuel injection management software with an oxygen sensor (lambda probe) in the exhaust system, for identifying the type of fuel in real time, made flexible-fuel engines viable.

By 2004, all car manufacturers in Brazil were offering flexible-fuel models and the increasing use of turbocharging proved ideal for these engines.

Since their introduction, flexible-fuel engines have been fitted to 30 million locally-made or fully imported models. Almost 90 percent of motorcycles, cars and light commercials on the domestic market are flexible-fuel powered.

All these vehicles are optimised to run on E20-E25 gasoline and up to 100-percent hydrous ethanol fuel (E100) – not the more refined and more expensive anhydrous ethanol.

The accompanying graph shows different fuel consumption by passenger vehicles around the world as at 2013. Note that only Europe and India are predominantly diesel-fuelled and, since 2013 governments in Europe and India have restricted diesel car usage. Sales of diesel cars in the EEC were down 26-percent
in 2018 and in 2019 had dropped to account for little more than a quarter of new vehicle registrations.

If Australia had gone down the Brazilian path in 2003 we could be enjoying low-cost, low-emissions, home-grown fuel today. We’d have a much healthier trade balance and certainty of supply.

Unfortunately, in Canberra, they’re too busy fighting each other to take note of progress around the world.

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