Fuel economy
The Difference Engine: Four bangs for the buck
LIKE millions of others, your correspondent’s extended family took to the road over the long weekend (Memorial Day in America, Spring Bank Holiday in Britain) that marks the unofficial start of summer. With warmer weather beckoning, the urge to migrate to the mountains, forests, beaches and deserts seems every bit as much a genetic imperative among humans as the migratory instincts of petrels, cetaceans or wildebeest.
From June to September, Americans have traditionally taken to the roads in droves, clocking up thousands of miles for leisure and pleasure. By all accounts, this year has started off no differently. Certainly, the nose-to-tail traffic leaving Los Angeles at the beginning of the holiday weekend testified to the perennial Whitsun wanderlust.
The surprise was the amount of traffic, given the current price of petrol. While the average pump price is down from the $4.11 peak reached just before the economy tanked in 2008, regular (ie, 87-octane) grade still costs $3.80 a gallon nationally. In California, which has some of the priciest petrol in the country, the average cost of a gallon of regular has fallen over the past week to $4.05 (€0.74/litre).
Forced to pay over twice as much, Europeans snigger at how little it costs even Californians to fill the tank. But people in the United States pay through the nose for many other things in life (health, education, property taxes, wireless services, etc) that foreigners get for far less or even free. For most Americans, having to drop $70 or more at a gas station can still give serious pause for thought about whether the journey is really necessary.
Yet, here’s the conundrum. Following all previous recessions, petrol consumption has been a leading indicator of recovery, bouncing back sharply as people started using their vehicles more to shop, to dine out, to seek the curious and the entertaining, and, above all, to take vacations. Despite the American economy’s belated and still timid recovery—seen in increasing sales of cars, clothing, hospitality, entertainment, and consumer goods generally (though still not housing)—the amount of petrol being consumed across the country has tumbled to 2001 levels, and shows every sign of falling further.
The Bureau of Economic Analysis, the federal agency that churns out monthly reports on how the economy is faring, believes the 2008 spike in petrol prices and the subsequent recession have changed the consumption patterns of American motorists irreversibly. How so? The short answer is that technology and marketing have altered the type of vehicles Americans are now buying.
For a start, the gas-guzzling V-8 engine that once ruled American roads has all but vanished from the showrooms. Starting imperceptibly in the early 1990s, American motorists traded in their eight-cylinder vehicles for the better fuel efficiency of six-cylinder models. Today, eight-cylinder cars and trucks account for less than 10% of new vehicles. In the new era of austerity, the hulking 6,000lb SUV, once the pride of the parking lot, is now ridiculed as obscene.
Meanwhile, sales of even six-cylinder vehicles have plummeted over the past five years, from 40% of the market to 25%. The big winners have been cars with four-cylinder engines, which have increased their share of the market from 48% five years ago to 65% today. In short, the new cars Americans are buying are almost as frugal as those driven by their counterparts in Europe, if not Japan.
From June to September, Americans have traditionally taken to the roads in droves, clocking up thousands of miles for leisure and pleasure. By all accounts, this year has started off no differently. Certainly, the nose-to-tail traffic leaving Los Angeles at the beginning of the holiday weekend testified to the perennial Whitsun wanderlust.
The surprise was the amount of traffic, given the current price of petrol. While the average pump price is down from the $4.11 peak reached just before the economy tanked in 2008, regular (ie, 87-octane) grade still costs $3.80 a gallon nationally. In California, which has some of the priciest petrol in the country, the average cost of a gallon of regular has fallen over the past week to $4.05 (€0.74/litre).
Forced to pay over twice as much, Europeans snigger at how little it costs even Californians to fill the tank. But people in the United States pay through the nose for many other things in life (health, education, property taxes, wireless services, etc) that foreigners get for far less or even free. For most Americans, having to drop $70 or more at a gas station can still give serious pause for thought about whether the journey is really necessary.
Yet, here’s the conundrum. Following all previous recessions, petrol consumption has been a leading indicator of recovery, bouncing back sharply as people started using their vehicles more to shop, to dine out, to seek the curious and the entertaining, and, above all, to take vacations. Despite the American economy’s belated and still timid recovery—seen in increasing sales of cars, clothing, hospitality, entertainment, and consumer goods generally (though still not housing)—the amount of petrol being consumed across the country has tumbled to 2001 levels, and shows every sign of falling further.
The Bureau of Economic Analysis, the federal agency that churns out monthly reports on how the economy is faring, believes the 2008 spike in petrol prices and the subsequent recession have changed the consumption patterns of American motorists irreversibly. How so? The short answer is that technology and marketing have altered the type of vehicles Americans are now buying.
For a start, the gas-guzzling V-8 engine that once ruled American roads has all but vanished from the showrooms. Starting imperceptibly in the early 1990s, American motorists traded in their eight-cylinder vehicles for the better fuel efficiency of six-cylinder models. Today, eight-cylinder cars and trucks account for less than 10% of new vehicles. In the new era of austerity, the hulking 6,000lb SUV, once the pride of the parking lot, is now ridiculed as obscene.
Meanwhile, sales of even six-cylinder vehicles have plummeted over the past five years, from 40% of the market to 25%. The big winners have been cars with four-cylinder engines, which have increased their share of the market from 48% five years ago to 65% today. In short, the new cars Americans are buying are almost as frugal as those driven by their counterparts in Europe, if not Japan.
The big difference today is that, in their adverts and marketing material, carmakers boast less about the number of cylinders their models possess, and more about the number of miles per gallon they get on the highway (figures for city driving are usually buried in the fine print). Last year’s version of the Ford Explorer, a three-row SUV, came with a four-litre V-6 engine that delivered a modest 210 horsepower and had an estimated (but rarely achieved) fuel economy of 20mpg (11.8litres/100km) on the highway. This year’s redesign can be ordered with a four-cylinder engine of just two-litres capacity that delivers 237 horsepower and apparently does a realistic 25mpg on the highway.
Meanwhile, BMW is about to start offering a four-cylinder engine in America for the first time in over a decade. The German carmaker’s new two-litre engine, which will be seen initially in its Z4 sportscar, is more powerful than the three-litre motor it replaces and 20% more fuel efficient.
What has turned the lowly four-banger into such a ready replacement for its bigger brethren is the widespread use of turbo-charging and direct fuel injection. Such power-boosting technologies have made it possible for the inherently “rough” straight-four to do the job of bigger, more refined motors. Unlike the better balanced straight-six or even the ultra-smooth V-12 engine, the humble four-cylinder generates an up and down vibration with a frequency of twice the crankshaft's rotational speed.
It’s an age old problem. In a four-cylinder engine, the forces caused by the primary harmonic of the crankshaft’s rotational frequency can be balanced by arranging one pair of pistons to move down while the other pair moves up. Unfortunately, the forces caused by the secondary harmonic remain out of kilter as a result of the pistons having to accelerate and decelerate faster in the upper half of the cylinders than in the lower half. For the secondary forces to be in balance, the rods connecting the pistons to the crankshaft would have to be of infinite length.
A practical, though expensive, solution was developed in 1904 by Frederick Lanchester, an inventor, engineer and poet, who was one of the founding fathers (with Harry Ricardo and Henry Royce) of the British motor industry. In Lanchester’s design, two balance shafts rotated in opposite directions at twice the crankshaft speed to cancel out the niggling secondary vibrations. Mitsubishi Motors was one of the first to popularise the use of balance shafts in straight-four engines built from the 1970s onwards.
Other makers of four-cylinder cars have either ignored the problem or adopted cheaper solutions—like heavier flywheels and more padded engine mounts to help dampen the secondary vibrations. But as the engine size increases, the vibration eventually becomes intolerable. The practical limit for an unbalanced inline-four is around 2.4 litres. That is why luxury cars have invariably used much smoother inline-six engines or V-8s—and why most of the new breed of four-cylinder motors rarely exceed two litres.
A lot of clever technology has gone into making the new four-cylinder engines cleaner and more efficient, if not smoother. One approach has been to use variable-lift inlet valves that can be raised or lowered to different heights, depending on the amount of power needed. Dispensing with the usual throttle arrangement in this way saves weight and complexity. It also minimises the pumping losses caused by no longer having to “throttle” (ie, strangle) the air flowing into the cylinders in order to control the engine speed.
Injecting fuel directly into the engine (rather than into the inlet manifold, upstream of the inlet valves) also has beneficial effects. Modern solenoid injectors can squirt very precise amounts of fuel close to the spark plug at 200 times atmospheric pressure, resulting in extremely clean and homogeneous combustion. Meanwhile, the cooling effects caused by high-pressure jets of fuel expanding rapidly into the cylinders allows the engine to operate at a higher compression ratio than normal. Adding all such improvements together can result in a 20% gain in fuel economy.
And that, ultimately, is what it is all about. Having to live with $4-a-gallon gas, Americans want cars that do 40mpg or more. But the carmakers want them even more so. The new federal fuel-economy standards will require motor manufacturers to achieve fleet-wide averages of 34.1mpg by 2016. Their task now is to convince customers—who they have long encouraged to believe that small engines mean underpowered vehicles that are not much fun to drive—to pay the same kind of money for frugal four-cylinder models that they once happily forked out for hulking big road-hogs.
What has turned the lowly four-banger into such a ready replacement for its bigger brethren is the widespread use of turbo-charging and direct fuel injection. Such power-boosting technologies have made it possible for the inherently “rough” straight-four to do the job of bigger, more refined motors. Unlike the better balanced straight-six or even the ultra-smooth V-12 engine, the humble four-cylinder generates an up and down vibration with a frequency of twice the crankshaft's rotational speed.
It’s an age old problem. In a four-cylinder engine, the forces caused by the primary harmonic of the crankshaft’s rotational frequency can be balanced by arranging one pair of pistons to move down while the other pair moves up. Unfortunately, the forces caused by the secondary harmonic remain out of kilter as a result of the pistons having to accelerate and decelerate faster in the upper half of the cylinders than in the lower half. For the secondary forces to be in balance, the rods connecting the pistons to the crankshaft would have to be of infinite length.
A practical, though expensive, solution was developed in 1904 by Frederick Lanchester, an inventor, engineer and poet, who was one of the founding fathers (with Harry Ricardo and Henry Royce) of the British motor industry. In Lanchester’s design, two balance shafts rotated in opposite directions at twice the crankshaft speed to cancel out the niggling secondary vibrations. Mitsubishi Motors was one of the first to popularise the use of balance shafts in straight-four engines built from the 1970s onwards.
Other makers of four-cylinder cars have either ignored the problem or adopted cheaper solutions—like heavier flywheels and more padded engine mounts to help dampen the secondary vibrations. But as the engine size increases, the vibration eventually becomes intolerable. The practical limit for an unbalanced inline-four is around 2.4 litres. That is why luxury cars have invariably used much smoother inline-six engines or V-8s—and why most of the new breed of four-cylinder motors rarely exceed two litres.
A lot of clever technology has gone into making the new four-cylinder engines cleaner and more efficient, if not smoother. One approach has been to use variable-lift inlet valves that can be raised or lowered to different heights, depending on the amount of power needed. Dispensing with the usual throttle arrangement in this way saves weight and complexity. It also minimises the pumping losses caused by no longer having to “throttle” (ie, strangle) the air flowing into the cylinders in order to control the engine speed.
Injecting fuel directly into the engine (rather than into the inlet manifold, upstream of the inlet valves) also has beneficial effects. Modern solenoid injectors can squirt very precise amounts of fuel close to the spark plug at 200 times atmospheric pressure, resulting in extremely clean and homogeneous combustion. Meanwhile, the cooling effects caused by high-pressure jets of fuel expanding rapidly into the cylinders allows the engine to operate at a higher compression ratio than normal. Adding all such improvements together can result in a 20% gain in fuel economy.
And that, ultimately, is what it is all about. Having to live with $4-a-gallon gas, Americans want cars that do 40mpg or more. But the carmakers want them even more so. The new federal fuel-economy standards will require motor manufacturers to achieve fleet-wide averages of 34.1mpg by 2016. Their task now is to convince customers—who they have long encouraged to believe that small engines mean underpowered vehicles that are not much fun to drive—to pay the same kind of money for frugal four-cylinder models that they once happily forked out for hulking big road-hogs.
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