The Future of the Internal-Combustion Engine

The Future of the Internal-Combustion Engine

Carlos Ghosn, the CEO of Nissan and Renault, has proclaimed that battery-powered vehicles will account for 10 percent of global new-car sales by 2020. Mr. Ghosn, of course, is planning to introduce at least four electric cars in the next three years. Independent analysts, however, such as Tim Urquhart of IHS Global Insight, believe that battery-powered vehicles will remain at less than one percent of the new-car mix in 2020.

The fact is that electric vehicles are prohibitively expensive today—the battery alone in an electric car can cost $20,000—and will remain so for some time. Moreover, electric vehicles are unproven in the real world. If carmakers are going to bet their futures on this technology, they will do so very gradually. Even under Ghosn’s optimistic view, internal-combustion (IC) engines will power 90 percent of 2020 vehicles. Koei Saga, Toyota’s boss of advanced technology (including electric cars), goes further: “In my personal view, I think we will never abandon the internal-combustion engine.”

But they won’t be the same IC engines that power vehicles today. With federal fuel-economy standards getting tougher by 35 percent over the next five years, IC efficiency must improve dramatically—if not, we’ll all be forced to drive econoboxes.

After speaking with key powertrain engineers and some independent inventors, we’ve examined some of the technologies that can achieve this improved efficiency.

The Future of the Internal-Combustion Engine

Spraying fuel directly into a gasoline engine’s combustion chambers instead of its intake ports isn’t a new idea—the World War II ME109 German fighter plane used it. The Japanese-market Mitsubishi Galant was the first car to combine direct injection with computer-controlled injectors in 1996. Direct injection (DI) costs more than port injection because the fuel is sprayed at 1500–3000 psi rather than 50–100 psi, and the injectors must withstand the pressure and heat of combustion.

But DI has a key benefit: By injecting fuel directly into the cylinder during the compression stroke, the cooling effect of the vaporizing fuel doesn’t dissipate before the spark plug fires. As a result, the engine is more resistant to detonation—a premature and near-explosive burning of the fuel, producing a knocking sound and pounding the pistons with pressure and heat—and can therefore operate with a higher compression ratio—about 12:1 instead of 10.5:1. That alone improves fuel economy by two to three percent.

The Future of the Internal-Combustion Engine

And DI also offers the possibility of lean combustion because the fuel spray can be oriented so that there is always a combustible mixture near the spark plug. That could yield five percent more efficiency.

Several European carmakers are already using this lean-burn strategy. Unfortunately, lean combustion causes higher tailpipe emissions of NOx (oxides of nitrogen), which run afoul of America’s tighter limits. Catalysts that can solve this problem don’t like the high sulfur content in American gasoline. New catalysts promise to reduce emissions. Meanwhile, expect direct injection to become universal by 2020.

The Future of the Internal-Combustion Engine

Modern engines achieve power levels that we could only dream about 20 years ago. The downside is that during routine driving, most engines are loafing—and 300-hp engines are inefficient when they’re only putting out the 30 ponies needed to push an average sedan down the highway. When an engine’s throttle is barely cracked open, there’s a strong vacuum in the intake manifold. During the intake stroke, as the pistons suck against this vacuum, efficiency suffers.

The classic solution to this problem is to make an engine smaller. A small engine works harder, running with less vacuum, and is consequently more efficient. But small engines make less power than big ones.

To make big-engine power with small-engine fuel economy, many companies are turning to smaller engines with turbochargers, direct fuel injection, and variable valve timing. These three technologies work together to their combined benefit.

Forcing additional air into an engine’s combustion chambers with a turbocharger definitely boosts power; car manufacturers have been doing this for years. But in the past, in order to avoid harmful detonation, turbocharged engines needed lower compression ratios, which compromised efficiency.

As we’ve seen, direct fuel injection helps solve this problem by cooling the intake charge to minimize detonation. Second, if the variable valve timing extends the time when both the intake and the exhaust valves are open, the turbocharger can blow fresh air through the cylinder to completely remove the hot leftover gases from the previous combustion cycle. And since the injectors squirt fuel only after the valves close, none of it escapes through the exhaust valve.

The first engine in America with all three of these elements was the base 2.0-liter four-cylinder in the 2006 Audi A4. It had a 10.5:1 compression ratio—as high as many naturally aspirated engines—despite a peak boost pressure of 11.6 psi. It produced 200 horsepower and 207 pound-feet of torque.

Ford’s EcoBoost system is nothing more than direct injection and turbocharging. Dan Kapp, Ford’s director of advanced powertrain engineering, says that this technology will spread across the company’s cars and trucks. “Nothing else delivers double-digit improvements in fuel efficiency at a reasonable cost.”

In the future, Ford expects to replace its 5.4-liter V-8 with a 3.5-liter EcoBoost V-6; its 3.5-liter V-6 with a 2.2-liter EcoBoost inline-four; and its 2.5-liter inline-four with a 1.6-liter EcoBoost inline-four. In each downsizing, peak power should be similar, low-end torque should be about 30 percent greater, and fuel economy should be 10-to-20 percent higher. The only downside will be an added charge of $1000 or so to the price of DI-turbo vehicles to pay for the additional hardware.

BMW, Mercedes, Toyota, and Volkswagen are planning similar engines—some using superchargers instead of turbochargers. Turbocharging with direct injection will continue to expand.

Later in the decade, we will see a second generation of these engines, using higher boost pressures. This will allow further engine downsizing to achieve an additional 10-percent efficiency improvement.

Making this happen will require cooled exhaust-gas recirculation to control detonation and either staged or variable-geometry turbos to limit customary lag. Those technologies are already in use on diesel engines, but a gas engine’s higher exhaust temperatures pose durability problems that must be solved before carmakers can deploy these technologies.

The Future of the Internal-Combustion Engine

Another way to improve the efficiency of a big engine is to turn off some of its cylinders. Since the throttle must be opened farther to get the same power from the remaining cylinders, intake-manifold vacuum goes down and efficiency goes up.

In real-world driving, this can produce a fuel-economy improvement of five percent, at a fairly low cost. The technology is particularly cost effective on pushrod, two-valve engines, which is why we’ve seen variable displacement on GM and Chrysler V-8s.

Honda uses variable displacement on its 24-valve V-6 engines, but the additional hardware to close the multiplicity of valves adds cost. Moreover, shutting off some cylinders on a V-6 generates more vibration and noise problems than it does with a V-8 because V-6s have coarser firing impulses and poorer inherent balance. The active engine mounts and variable intake manifolds needed to solve these problems add further costs.

The Future of the Internal-Combustion Engine

The simplest implementation of variable valve timing started about 25 years ago, using a two-position advance or retard of either an engine’s intake or exhaust camshaft to better match the engine’s operating conditions. Today, most four-valve-per-cylinder DOHC engines have continuously variable phasing on both the intake and the exhaust camshafts.

About 20 years ago, Honda introduced a more elaborate approach with its VTEC system, which shifted between two (and later, three) separate sets of cam lobes—one for high-speed operation and one for low. VTEC can also simply turn off one of a cylinder’s two intake valves under light loads. In 2001, BMW went a step further with its Valvetronic system, which can continuously vary the opening stroke of the intake valves to optimize engine power and efficiency. Furthermore, this extensive control of the intake valves serves to replace a throttle plate, which eliminates vacuum and therefore reduces pumping losses.

Though they provide efficiency benefits, variable-lift systems are complex and expensive. Development continues on purely electronic systems that could replace camshafts and simply open and close an engine’s valves according to a computer. But electronic valve-opening mechanisms are also costly and consume significant power. GM Powertrain VP Dan Hancock suggests that a two-stage valve-lift mechanism can deliver 90 percent of the benefits of fully variable lift. Moreover, Ford’s Kapp says that the benefits of variable valve lift are limited when combined with EcoBoost (DI turbo).

On the other hand, BMW, with its latest single-turbo, direct-injection 3.0-liter inline-six (N55) that’s replacing the twin-turbo (N54) across the lineup, has done just that by adding Valvetronic to its DI-turbo configuration. Combined with the move from a six-speed automatic to an eight-speed, the change is said to provide 10 percent more miles per gallon.

The Future of the Internal-Combustion Engine

Perhaps the answer will be Fiat’s Multiair system, a hydraulically operated variable-lift design that is far less complex than mechanical systems such as BMW’s. Expect to soon see Multiair on upcoming Chrysler vehicles.



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