Thursday, 27 July 2017

Forced Air and the Future

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Nothing like a little mouth to mouth to get you breathing deep

Forced induction. These two words together can make a grown man salivate in a way that not many other things can. What this term refers to, in relation to the function of the automobile, is any device or method that forces more air into the intake of an engine. Forcing more air into the engine means that more fuel can be burned when needed, and more power can be produced by the engine when needed.

Many people think that the force that causes air to flow into the engine is the suction created by the piston moving down in the cylinder with the intake valve open. This is not true. The piston moving downward only creates vacuum, which is an area of negative pressure. Once this void is created something will rush in to fill it, and that something is air. The motivation for the air is atmospheric pressure. This is one of the reasons that the average car or truck will produce more power at low elevation versus high elevation.

paxton supercharger

A centrifugal type supercharger.

At sea level normal air pressure is 14.7 psi. This means that a column of air at sea level measuring one square inch, reaching all the way up through the atmosphere, weighs 14.7 pounds. The further up from sea level that you measure the weight of the column, the less it will weigh. In Denver, CO the column of air will weigh about 12.2 pounds. In Leadville, CO which is a town over 10,000 ft. high in the Rockies, the column will weigh 10.3 pounds. Because of this decrease in pressure, a car that is operating at this elevation will not be as powerful. The other factor affecting automotive aspiration at elevation, which is related to the first, is the fact that there is just less oxygen to begin with because of the lower pressure.

In the early days of modern aviation the service ceiling of the average aircraft was limited by this same fact. If an airplane flew too high it would lose power. Forced induction systems first showed up on aircraft during WWI. This allowed the planes to fly much higher than they otherwise could have. Like so many other bits of technology that improved aircraft, it was only a matter of time before some mechanic said, “I think I should try bolting one of these things on my car.”

Types

Forced induction systems can be divided into two distinct groups, turbochargers and superchargers. In reality a turbocharger is a type of supercharger but usually the two do not get lumped together into the same category because they don’t operate the same way, even though they essentially accomplish the same task.

supercharged engine

A large supercharger on top of a 5.3L Chevy V8.

The supercharger is a belt driven compressor that forces large amounts of air into the intake manifold. A belt similar to the type that turns accessories such as the alternator or the A/C compressor will be used to turn a set of screws within a housing that are in mesh with each other. Superchargers can take on many different forms but they are always belt driven. The amount of pressure that a supercharger will build varies from one engine to another but it is usually somewhere between 8 and 15 psi.

The turbocharger compresses air and forces that compressed air into the intake manifold, but it is driven by exhaust gasses exiting the engine through the exhaust manifold. These gasses get pushed out towards the tailpipe and as they go they pass though the turbo. They enter the turbine housing where they spin a wheel that is attached to a shaft. On the other end of the shaft is another wheel called the compressor that compresses the air that is being pushed into the engine through the air filter.

Turbochargers will produce boost pressure in the same neighborhood as superchargers do, but they do not drag on the engine because they are not driven by the crankshaft. This sounds like a great advantage for the turbo but not everything is magic and rainbows. A turbo will cause an increase in exhaust back pressure, which technically affects engine power, and there is also the issue of turbo lag. This is a condition of brief delay between the time the noisy pedal hits the floor, and the time the engine responds with all of its power. This delay is caused by the turbo taking a moment to reach its maximum rotational speed to generate the proper level of boost. Modern technology has really come a long way to minimize turbo lag, and no doubt will continue to do so.

Most engines that use some kind of forced induction system will incorporate an intercooler between the compressor device and the intake manifold. The intercooler cools the compressed air charge after it exist the compressing mechanism. Because the turbocharger or supercharger increases the pressure of the intake air, it will also increase the temperature. Temperature and pressure are always tied together this way. If the temperature is high then the density of the air, more specifically the oxygen, will be lower. So the intercooler increases oxygen density by cooling the air. This makes the system more efficient.

The Future

In the past, any vehicle with a forced induction system was considered high performance, or at least higher performance then the equivalent vehicle in naturally aspirated form. A forced induction system fitted to an engine that could already be considered high performance makes for a real screamer. Top Fuel dragsters have massive superchargers breathing air into massive engines. These cars can run a quarter of a mile in about 4 seconds. A tremendous amount of air is forced into the engine in order to burn a tremendous amount of fuel. In the real world we only need lots of power for brief moments here and there. Like when we are getting on the freeway, passing a slower vehicle on a two lane road, or when we are trying to beat another car to a prime parking spot at the mall. The rest of the time we only need a small amount of power to get around. A full-size pickup truck only uses about 20 to 25 horsepower to maintain 50 mph on a flat road.

2011 Ford F-150 3.5-liter EcoBoost Engine

Ford Ecoboost 3.5L V6. This engine is taking the world by storm. If they get a little better fuel economy out of it this engine will be really impressive.

With all of the talk about new automotive technologies that are very advanced and becoming more widely available, the one that is just starting to get more attention is the new turbocharged engines, and the high levels of efficiency that they can obtain. They haven’t been talked about too much until recently because the technology is really not that new, and turbocharged offerings have only recently become very abundant. The new turbocharged engines of today use the technology most often for efficiency and not for crazy mind bending performance.

With the proper application of a turbocharger, an engine’s output and efficiency can be raised enough to make the engine seem like something much bigger than it really is, but only when it’s needed. 4 cylinder engines can easily have the power of a V6, and a V6 can have the power of a V8. This can happen without sacrificing fuel economy because small displacement engines are used. Using a variable geometry turbos (an explanation of this is the stuff of another column), the engine can be efficient at low speeds and produce big power under heavy load. For all the times of normal operation, the smaller size helps it get better fuel economy.

The Ford Ecoboost engines are a great example of an engine family that puts out tremendous power when needed but still gets great fuel economy. Ford produces multiple V6 and I4 Ecoboost engines that are offered in multiple vehicles. The 4 cylinder engines act like 6 cylinder engines under heavy load, and the 6 cylinder engines act like 8 cylinder engines under load. The new Ford GT has a version of the 3.5L V6 Ecoboost that is expected to be good for more than 600 HP. The Ford F150 is available with a 3.5L or 2.7L V6 and these engine will outsell the 5.0 V8. Is the V8 as we know it on death’s door?

cruze turbo

Other vehicles offering turbos for the sake of efficiency include the Chevrolet Cruze, Hyundai Sonata, Dodge Dart, Volkswagens of all flavors, certain Mazdas, and many more. Many of the smallest turbocharged engines are able to achieve fuel economy that is very close to what a hybrid is able to achieve but the extra cost of the turbo is much less than the extra cost of a hybrid system.

Why isn’t everyone using these kind of forced induction systems on all of the vehicles they build? The biggest reason is cost. While a small turbo setup might be much cheaper than a hybrid drivetrain, it still costs more than an engine that doesn’t have a turbo. As fuel economy standards continue to tighten, all of the auto manufacturers will have to adapt some way or another. Also, gas prices being volatile such as they are, the public will demand more efficient vehicles that get better gas mileage every time the price of gas goes up. Maybe smaller engines with forced induction systems will be the best answer for all of them.

DSCN6981

A very healthy looking Ford F150 with the 3.5 Ecoboost.

Some express mixed ideas on the benefits of this movement towards turbos. Some purists and automotive enthusiasts feel like a turbocharged engine does not deliver power in a desirable manner, they say the power flow is not linear, or that there is no way to eliminate turbo lag effectively. They also say that the constant boosting and heat loading in the turbo, intake, and combustion chamber is not good for engine longevity. Some of these points are valid but none of them are issues that cannot be overcome with more technology and better engineering.

Some say turbocharged vehicles don’t get good fuel economy because people tend to goose the throttle until the boost hits hard. This is probably true as well. That’s okay, they’re just having fun, and considering the fact that turbochargers are becoming so common, people might get used to this eventually as well.

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