Smaller, more efficient engines better matched to more complex transmissions will have some of the biggest effects in making tomorrow's cars more fuel-efficient.

The 2025 corporate average fuel economy (CAFE) level of 54.5 mpg translates to about 42 mpg on the EPA window sticker.

And while Toyota Prius hybrid owners may note that they can do better than that today, the average applies to all vehicle types--not just small cars but even full-size pickup trucks.

Reduced weight and lower wind resistance will play a role in getting your next vehicle to deliver better gas mileage than ever before, no matter how large or small they are.

And many of the changes will come in engines and transmissions.

This is the second of three articles on the future technologies that will boost fuel efficiency in future cars.

So what will those changes consist of?

SMALLER, MORE EFFICIENT GASOLINE ENGINES

Engine efficiency improvements may be the single biggest factor in improving gas-mileage ratings for the next few years.

A variety of technologies--direct injection, variable valve timing and valve lift, even electrically actuated valves--are already making combustion more efficient.

That permits what's known as "downsizing": an engine, frequently with fewer cylinders, that's smaller, lighter, and has fewer moving parts--which reduces fuel-wasting internal friction.

To make up for fewer cylinders, many of these engines are turbocharged--using otherwise wasted exhaust heat to power a pump that packs more air into the cylinder.

The resulting higher air pressure can be combined with more gasoline, meaning the engine produces more power on combustion.

That gives these engines higher horsepower for their size than "regularly aspirated" engines, meaning a four can perform like the V-6 it replaces.

In other words: Don't count the gasoline engine out for many years to come.

Example: EcoBoost

The best example of this is Ford's line of EcoBoost engines, which started with a 3.5-liter turbocharged V-6 with the same power and towing capacity as the larger V-8 it replaced in Ford's best-selling F-150 pickup truck.

That engine has been a surprise, selling much better than Ford expected, especially in full-size pickup trucks.

It was joined by a 2.0-liter four that replaces a 3.0- to 3.5-liter V-6, and then by both 1.5- and 1.6-liter fours that replace larger standard 2.5-liter fours.

Most recently, Ford has added a 1.0-liter three-cylinder turbocharged engine, which replaces a conventional four of up to 1.6 liters.

And, who knows, there may be more to come.

The downside of smaller turbo engines is that they're more economical under light load than the bigger ones they replace, but equally thirsty at high power.

So aggressive drivers may find them thirstier than their EPA ratings, while gentler souls do just fine.

Few V-8s outside trucks

By 1970 or so, even so-called compact sedans offered V-8 engines. The only sixes were on base models, and four-cylinder engines from U.S. makers were a rarity.

2011 Hyundai Sonata 2.0T

2011 Hyundai Sonata 2.0T

The rapid market gains achieved by smaller, more fuel-efficient, and better-built Japanese cars taught Detroit a 20-year lesson.

Now, if anything, some categories of U.S. cars use smaller engines than their Japanese competitors.

Big V-8s these days are limited to full-size pickup trucks--all of which come with V-6es as the base engine now--and a handful of pony cars and high-end large sedans.

The V-6 has effectively taken the place of the V-8, but it's really four-cylinder engines that are now the norm.

Andthey can be much larger than the 1.2- to 1.6-liter "econobox" engines that some buyers still imagine them to be.

Fours are the default

Around 2010, makers began to get rid of their V-6 options in both mid-size cars and compact crossovers, and the trend has accelerated rapidly.

2014 Chevrolet Impala, test drive in Hell, Michigan

2014 Chevrolet Impala, test drive in Hell, Michigan

Even some full-size cars now offer four-cylinder engines: the 2014 Chevrolet Impala comes standard with a 196-hp, 2.5-liter four.

Not so many years ago, it might have been a base V-8--certainly a V-6--that returned 200 hp.

But the new 2014 Impala only comes with four- or six-cylinder engines; no V-8 is available.

Nor is GM likely to offer one: Those are confined to Camaros, police vehicles, and trucks.

V-6 midsize sedans: Only a few left

Among mid-size sedans, the fours that used to be found in compact cars are now ubiquitous.

Sixes are no longer offered in the Chevrolet Malibu, Ford Fusion, Hyundai Sonata, or Kia Optima--though they're still a high-end option on the Honda Accord, Chrysler 200, Nissan Altima, Toyota Camry, and Volkswagen Passat.

The Volkswagen Passat is odd man out in that group, by the way; its base engine is not a four, but a 2.5-liter five-cylinder.

With a new line of smaller, more powerful turbocharged fours coming from a new Mexican engine plant, future Passats will likely fall in line.

Similarly, the popular compact crossover segment has three competitors without V-6 options--most notably the 2013 Ford Escape, along with the Hyundai Santa Fe and Kia Sportage. And there will likely be more in years to come.

Smaller sizes sans turbos

Even without turbochargers, fours are getting smaller.

Ford's 1.0-Liter EcoBoost Engine

Ford's 1.0-Liter EcoBoost Engine

Mazda has replaced a 2.5-liter four with a new, highly efficient 2.0-liter 'SkyActiv' engine that combines a high combustion ratio with complex and carefully tuned manifolds like those on racing engines to boost efficiency.

It works, too: We tested two different SkyActiv models last year, a 2013 Mazda CX-5 crossover and a 2012 Mazda3 hatchback, and both handily exceeded their EPA ratings.

Coming soon: three cylinders

And three-cylinder engines are coming soon for subcompacts and even compact cars, from brands as diverse as BMW and MINI , Ford, and Mitsubishi.

Like all the downsized engines, they'll have the same power as the larger ones they replace.

Ford's award-winning 1.0-liter EcoBoost three will arrive in the 2014 Fiesta as an optional engine.

Ford says the three-cylinder Fiesta model will have among the highest fuel-economy ratings of any non-hybrid car on the market.

Downside of downsizing

There's not always a free lunch, though. Smaller turbocharged engines are often more expensive to make than larger, much simpler engines of the same power.

2012 Mazda Mazda3 SkyActiv

2012 Mazda Mazda3 SkyActiv

And when conventional engines get smaller--that four-cylinder Impala, for example, or the otherwise excellent 2.0-liter version of the Mazda CX-5 crossover--performance can suffer under some conditions.

We've driven a few new cars lately that are fine around town, tuned to be responsive away from stoplights, and otherwise acceptable in most uses.

But in the most extreme cases--a short, uphill freeway on-ramp merging into fast traffic with five people and their luggage in the car--there's just not the power there that a bigger engine provides.

Performance has increased fairly steadily over the last 20 to 30 years; you might be shocked at how slow even a more powerful car from 1993 actually is. Today's compacts are often capable of out-accelerating full-size sedans from the 1980s.

Missing top-end power

But drivers may still have to get used to the missing top-end power, requiring them to plan ahead and accelerate harder, earlier to achieve the same effect.

2013 Mazda CX-5 road test, Catskill Mountains, NY, April 2012

2013 Mazda CX-5 road test, Catskill Mountains, NY, April 2012

Will the resulting fuel economy be worth it?

We were pleasantly shocked when the 2013 Mazda CX-5 with the 2.0-liter SkyActiv engine returned more than 33 mpg in real-world usage, against its EPA combined rating of 28 mpg.

The same engine in a Mazda3 compact hatchback did even better, giving us a jaw-dropping 38.1 mpg over the same route--far better than its 33-mpg rating.

But their acceleration from, say, 60 to 80 mph was more anemic than it would have been with a larger engine.

You have to decide for yourself: Are gas-mileage numbers like that worth sacrificing a little top-end power?

Stop-start systems too

For many people, it's a sign of trouble if the engine dies at a stoplight.

But an idling engine at a stoplight that can last up to 2 minutes is the purest waste of fuel possible.

European and Asian buyers are much further along in accepting so-called "stop-start" systems, which merely switch off the engine when the car comes to a stop, then switch it on again when the driver starts to lift off the brake (or lets in the clutch on manual-gearbox cars).

These are not hybrid systems, but simply beefier batteries and starter motors with some electronics.

Now, cars are increasingly being modified to work with stop-start systems that can run electrically-driven ventilation systems on battery power for a minute or two, making the interruption less apparent.

No use at highway speeds

The systems save nothing at highway speeds, which is one reason they've been slow to arrive in the States.

2013 Ram 1500 - engine start-stop system

2013 Ram 1500 - engine start-stop system

But for cars used in urban and suburban stop-and-go traffic, the improvement can be as high as 10 or 12 percent.

Future iterations might even permit a slight "idle creep" to let cars move forward in very slow traffic--almost like a hybrid, minus the expensive high-voltage battery--without having to switch on the engine every time forward motion is needed.

Because U.S. drivers spend more time on highways and less in urban congestion, car companies have been more cautious in introducing start-stop systems to the U.S.

There's also the problem that they produce only minimal increases in the EPA city test cycle, which has very few long waits with the car stopped--yet another case where EPA test procedures vary from real-world use.

The first start-stop systems have only shown up in the last year or two, among vehicles as diverse as the Kia Rio, Porsche Panamera, and Ram 1500 pickup truck.

DIESEL ENGINES for PASSENGER VEHICLES

We'll also see more modern clean diesel engines fitted to passenger cars.

Diesels could really be the topic of their own article, but suffice it to say that diesels are coming (and coming back) from makers that haven't offered them in decades, or ever.

They join Volkswagen and Mercedes-Benz, brands that have been the diesel stalwarts in the U.S. market for decades.

2012 Volkswagen Passat TDI Six-Month Road Test

2012 Volkswagen Passat TDI Six-Month Road Test

Last year, VW built the vast majority of the diesel cars sold in the U.S. It now offers its 2.0-liter TDI turbodiesel four in its Beetle, Golf, and Jetta compacts, and the Passat mid-size sedan, as well as the pricey Touareg luxury sport-utility vehicle.

Similarly, Mercedes-Benz is fitting diesels not only to its ML and GL crossover utilities, but also to the S Class and E Class sedans and, most recently, the GLK compact crossover.

Audi and BMW joined their fellow Germans a few years ago, and Audi will offer diesels throughout its range--especially when the 2015 Audi A4 TDI arrives.

New diesel entries

The three latest arrivals are Chevrolet, Jeep, and Mazda.

The 2014 Chevy Cruze Diesel is is now on sale, and the 2014 Jeep Grand Cherokee EcoDiesel will arrive soon. The 2014 Mazda6 diesel is expected later in the year as well.

2014 Chevrolet Cruze Diesel, test drive in Hell, Michigan

2014 Chevrolet Cruze Diesel, test drive in Hell, Michigan

The Cruze and Grand Cherokee, in particular, are high-volume vehicles. That means that even a small percentage of diesel sales--10 percent, plus or minus--could notably increase the numbers of diesels sold in the U.S.

While it appears unlikely that 10 percent of U.S. vehicle sales will be diesels by 2015, as Bosch notably predicted (the company sells diesel equipment), their future is brighter now than in many years.

Real-world MPG better than EPA

Diesels have the added advantage that they frequently return real-world mileage higher than their EPA ratings, especially in highway use.

That's in contrast to some recent hybrids that haven't come close to hitting their EPA numbers this year.

The longer term outlook for diesels, however, remains cloudy.

They're 10 to 15 percent more expensive to build than a gasoline engine of the same output, and increasingly stringent emission requirements mean that exhaust aftertreatment systems must be more and more complex.

Open Q: Will diesels do well?

Mazda and some VW models aside, modern clean diesels must use urea injection into the catalyst to make the chemistry for lowering emissions come out properly.

(No, you can't pee in the tank to replace the urea fluid--though you'd be surprised how often that question is asked.)

2014 Jeep Cherokee

2014 Jeep Cherokee

Today, it's one of the open questions in the auto industry: Will U.S. buyers take to diesels in even a fraction of the numbers that Europeans have?

We suspect that car buyers, over time, will adopt diesels for those uses that favor long stretches of high-speed driving (the "traveling salesman" pattern).

Hybrids (which we'll cover in the next article), on the other hand, will continue to be used for those applications that require lots of stop-and-start driving (the "NYC cabbie" pattern).

TRANSMISSIONS

Just a few years ago, it was news when carmakers started to replace four-speed automatic transmissions with six-speed models.

Eight- and nine-speed automatics

Now, the six-speed is pretty much standard, and eight-speed automatics (once limited solely to Lexus) are about to appear in mid-size and even compact vehicles.

Chrysler, in fact, is planning to launch a nine-speed automatic in its upcoming 2014 Jeep Cherokee compact crossover. The same transmission will be used in pricey Range Rover Evoque models as well.

The other advance will be more use of continuously-variable transmissions (CVTs), which don't have fixed gear ratios but instead can vary the drive ratio widely to match the most fuel-efficient engine speed.

Ford tried CVTs in its Five Hundred sedan and Freestyle wagon almost 10 years ago, and quickly dumped them. Nissan to date has been the only real proponent of CVTs.

More 'natural' CVTs

But that's changing, as powerhouse cars from Honda (its 2013 Accord) and Toyota (its 2014 Corolla) come with CVTs. Smaller Subaru, too, is in the process of replacing all but a handful of its old-style automatics with its new LinearTronic CVT.

For drivers, the new breed of CVT is much less annoying than those of even a decade ago. When accelerating, the transmissions are programmed not to vary so much that the engine revs up to a high level and stays there.

Honda CVT transmission

Honda CVT transmission

Many of the latest CVTs also have the ability to mimic the gear progression of an old-style automatic when the driver wants--for temporary speed or just because drivers like to shift.

It's all done through control software, but the goal is wring out the lowest fuel usage while keeping the driver in the dark about the different type of transmission.

In the newest CVT-equipped cars--the Honda Accord and the Subaru Impreza, for instance--you can hear the engine note rising or falling even at a constant road speed, but acceleration from rest is much closer to the "normal' feel of a regular automatic.

PREVIOUSLY ...

Part I: New Gas Mileage Rules Will Reshape What Americans Drive: Aerodynamics And Weight

NEXT ...

Part III: New Gas Mileage Rules Will Reshape What Americans Drive: Hybrids, Electrics, Costs

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