Improvements in Engine Design- 3 On-board Engine Computers

Benefits: Fuel economy, better diagnosis of problems

Drawbacks: Cost, complexity

An engine is an incredibly sophisticated device. It has dozens of moving parts and has scores of different processes taking place at once. That's why modern cars have everything regulated by an on-board computer called an engine control unit, or ECU.

The ECU makes sure processes like ignition timing, the air/fuel mixture, fuel injection, idle speed, and others operate the way they're supposed to. It monitors what's going on in the engine using an array of sensors and performs millions of calculations each second in order to keep everything operating correctly. Other computers in the car control things like electrical systems, airbags, interior temperature, traction control, anti-lock brakes and the automatic transmission.

Cars have become increasingly computerized since the first on-board diagnostic (OBD) computers were added in the 1980s. That's the computer that's responsible for the "check engine" light on your dashboard. A mechanic can plug a computer into the OBD port and get a sense of your car's problem areas. They can't use OBD to immediately know what's wrong with your car, but it gives them a great starting point.

By making the engine run more efficiently, engine computers can result in greater fuel efficiency and easier diagnosis of problems. But they also make engines far more complicated, and can make them tricky for weekend mechanics to work on.

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Improvements in Engine Design -2 Clean Diesel

Benefits: Torque, fuel economy, cleaner emissions

Drawbacks: Cost of fuel, low RPMs, higher initial cost

We've talked a lot about gasoline engines so far, but what about diesel engines? Diesels have never been big sellers in the United States. Despite their superior fuel economy over similar gas engines, many Americans still think of diesels as the noisy, sooty, smelly, unreliable motors of the 1970s and 1980s.

That's not the case anymore. The modern diesel engine is powerful, clean and extremely fuel-efficient. Today's engines use a low-sulfur form of diesel fuel, and systems within the car help eliminate particle matter and excess pollution.

The diesels made by companies like Volkswagen, Mercedes-Benz, BMW, Volvo and others boast engine improvements like turbocharging, sophisticated fuel injection, and computer control to provide a driving experience that's both efficient and high in torque [source: Bosch].

Diesel engines have some drawbacks, mainly their low RPM level and the higher cost of diesel fuel. But since many of them can achieve well over 40 miles per gallon (17 kilometers per liter) on the highway, the driver will need to pay for that fuel a lot less often. And if you're wondering if modern diesels offer good performance, look no further than the last few 24 Hours of Le Mans races, where Audi has dominated using a diesel racecar.

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Improvement in Engine Design- 1 Hybrid Engines

Benefits: Fuel economy

Drawbacks: Higher initial cost, complexity

A combination of high gas prices, an increased awareness of the environment among drivers, and government regulations raising fuel economy and emissions standards have forced engines to "go green" more than ever before. One of the biggest engine improvements used to boost efficiency in recent years is the hybrid engine.

Hybrids were an obscure a decade ago, but now everyone knows how they work -- an electric motor is partnered with a traditional gasoline engine in order to achieve high fuel economy numbers, but without the "range anxiety" of an electric engine, where the driver always wonders what will happen when a charge runs out.

The Toyota Prius remains the top selling hybrid car in America. It boasts a 1.8-liter four cylinder engine coupled with an electric motor that produces 134 horsepower. At low speeds, the electric engine acts alone, meaning the car does not use gas at all. At other times, it assists the gasoline engine. The whole package gets about 50 miles per gallon (21.3 kilometers per liter) in both the city and the highway [source: AOL Autos].

Hybrids like the Prius represent the latest evolution in internal combustion technology. While their benefits come in the form of fuel efficiency, there are some drawbacks as well. Hybrids have a higher initial cost than their non-hybrid counterparts, and some have argued that gas must be much more expensive than it is now (unbelievable as that may sound) before the driver recoups the extra cost of the hybrid car.

However, it's clear that engines are trending towards reduced emissions and greater fuel-efficiency. While electric-only cars are becoming more common, it's clear the internal combustion engine isn't going anywhere quite yet. It will simply continue to evolve to be better and better, just like it has since the days of the Model T.

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Four-Stroke Combustion Cycle

Introduction

We all get into our car, turn the key in the ignition, step on the gas and off we go. But have you ever wondered how a car engine works? The principle behind how the engine works is that if you put a little bit of high-energy fuel, such as gasoline, into a small, enclosed space and add a spark, a huge amount of energy will be released in the form of expanding gas. When a cycle of hundreds of these reactions happens every minute, this energy can be used to run a car. Currently, most cars on the market use the four-stroke combustion cycle (invented by Nikolaus Otto in 1867, so sometimes it's referred to as the Otto cycle). The four strokes in the Otto cycle are the intake stroke, the compression stoke, the combustion stroke and the exhaust stroke.

As the crankshaft revolves, it moves the piston down, and the intake valve opens to let in a mixture of air and fuel (this is the intake stroke). When the piston moves back up, it compresses the air/fuel mixture (compression stroke), so when the spark plug releases a spark, the resulting explosion of fuel pushes the piston back down (combustion stroke). When the piston gets to the bottom of the stroke, the exhaust valve then opens, and when the piston moves up, it pushes the exhaust from the cylinder out through the car's tailpipe (the final stroke in the cycle – the exhaust stroke).

The engine is now ready for the next cycle to begin, so the intake valve opens and another batch of air with a tiny bit of gas is drawn into the cylinder.

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Disadvantages of the Two-stroke

You can now see that two-stroke engines have two important advantages over four-stroke engines: They are simpler and lighter, and they produce about twice as much power. So why do cars and trucks use four-stroke engines? There are four main reasons:

1. Two-stroke engines don't last nearly as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear a lot faster.
2. Two-stroke oil is expensive, and you need about 4 ounces of it per gallon of gas. You would burn about a gallon of oil every 1,000 miles if you used a two-stroke engine in a car.
3. Two-stroke engines do not use fuel efficiently, so you would get fewer miles per gallon.
4. Two-stroke engines produce a lot of pollution -- so much, in fact, that it is likely that you won't see them around too much longer. The pollution comes from two sources. The first is the combustion of the oil. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit huge clouds of oily smoke. The second reason is less obvious but can be seen in the following figure:

Each time a new charge of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port. That's why you see a sheen of oil around any two-stroke boat motor. The leaking hydrocarbons from the fresh fuel combined with the leaking oil is a real mess for the environment.

These disadvantages mean that two-stroke engines are used only in applications where the motor is not used very often and a fantastic power-to-weight ratio is important.

In the meantime, manufacturers have been working to shrink and lighten four-stroke engines, and you can see that research coming to market in a variety of new marine and lawn-care products.

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Two-stroke Engines Work

Sparks Fly

You can understand a two-stroke engine by watching each part of the cycle. Start with the point where thespark plug fires. Fuel and air in the cylinder have been compressed, and when the spark plug fires the mixture ignites. The resulting explosiondrives the piston downward. Note that as the piston moves downward, it is compressing the air/fuel mixture in the crankcase. As the piston approaches the bottom of its stroke, the exhaust port is uncovered. Thepressure in the cylinder drives most of the exhaust gases out of cylinder, as shown here:

Fuel Intake
As the piston finally bottoms out, the intake port is uncovered. The piston's movement haspressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel, as shown here:

Note that in many two-stroke engines that use a cross-flow design, the piston is shaped so that the incoming fuel mixture doesn't simply flow right over the top of the piston and out the exhaust port.


The Compression Stroke

Now the momentum in the crankshaft starts driving the piston back toward the spark plug for thecompression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor.

Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle. It's called a two-stoke engine because there is a compression stroke and then a combustion stroke. In a four-stroke engine, there are separate intake, compression, combustion and exhaust strokes.

You can see that the piston is really doing three different things in a two-stroke engine:
On one side of the piston is the combustion chamber, where the piston is compressing the air/fuel mixture and capturing the energy released by the ignition of the fuel.
On the other side of the piston is the crankcase, where the piston is creating a vacuum to suck in air/fuel from the carburetor through the reed valve and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber.
Meanwhile, the sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.

It's really pretty neat to see the piston doing so many different things! That's what makes two-stroke engines so simple and lightweight.

If you have ever used a two-stroke engine, you know that you have to mix special two-stroke oil in with the gasoline. Now that you understand the two-stroke cycle you can see why. In a four-stroke engine, the crankcase is completely separate from the combustion chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings on either end of the piston's connecting rod and the cylinder wall. In a two-stroke engine, on the other hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it can't hold a thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. If you forget to mix in the oil, the engine isn't going to last very long!

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Two-stroke Engines

This is what a two-stroke engine looks like:

You find two-stroke engines in such devices as chain saws and jet skis because two-stroke engines have three important advantages over four-stroke engines:
Two-stroke engines do not have valves, which simplifies their construction and lowers their weight.
Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost.
Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem can add complexity to the engine.

These advantages make two-stroke engines lighter, simpler and less expensive to manufacture. Two-stroke engines also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combination of light weight and twice the power gives two-stroke engines a great power-to-weight ratio compared to many four-stroke engine designs.

You don't normally see two-stroke engines in cars, however. That's because two-stroke engines have a couple of significant disadvantages that will make more sense once we look at how it operates.

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Ignition System Distributor

The distributor handles several jobs. Its first job is to distribute the high voltage from the coil to the correct cylinder. This is done by the cap and rotor. The coil is connected to the rotor, which spins inside the cap. The rotor spins past a series of contacts, one contact per cylinder. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder. When you do a tune-up, one of the things you replace on your engine is the cap and rotor -- these eventually wear out because of the arcing. Also, the spark-plug wires eventually wear out and lose some of their electrical insulation. This can be the cause of some very mysterious engine problems.

Older distributors with breaker points have another section in the bottom half of the distributor -- this section does the job of breaking the current to the coil. The ground side of the coil is connected to the breaker points.

A cam in the center of the distributor pushes a lever connected to one of the points. Whenever the cam pushes the lever, it opens the points. This causes the coil to suddenly lose its ground, generating a high-voltage pulse.

The points also control the timing of the spark. They may have a vacuum advance or a centrifugal advance. These mechanisms advance the timing in proportion to engine load or engine speed.

Spark timing is so critical to an engine's performance that most cars don't use points. Instead, they use a sensor that tells the engine control unit (ECU) the exact position of the pistons. The engine computer then controls a transistor that opens and closes the current to the coil.

In the next section, we'll take a look at an advance in modern ignition systems: the distributorless ignition.

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Ignition System Timing

The spark plug fires before the piston reaches top dead center.

The ignition system on your car has to work in perfect concert with the rest of the engine. ­The goal is to ignite the fuel at exactly the right time so that the expanding gases can do the maximum amount of work. If the ignition system fires at the wrong time, power will fall and gas consumption and emissions can increase.

When the fuel/air mixture in the cylinder burns, the temperature rises and the fuel is converted to exhaust gas. This transformation causes the pressure in the cylinder to increase dramatically and forces the piston down.

In order to get the most torque and power from the engine, the goal is to maximize the pressure in the cylinder during the power stroke. Maximizing pressure will also produce the best engine efficiency, which translates directly into better mileage. The timing of the spark is critical to success.

There is a small delay from the time of the spark to the time when the fuel/air mixture is all burning and the pressure in the cylinder reaches its maximum. If the spark occurs right when the piston reaches the top of the compression stroke, the piston will have already moved down part of the way into its power stroke before the gases in the cylinder have reached their highest pressures.

To make the best use of the fuel, the spark should occur before the piston reaches the top of the compression stroke, so by the time the piston starts down into its power stroke the pressures are high enough to start producing useful work.

Work = Force * Distance

In a cylinder:
Force = Pressure * Area of the piston
Distance = Stroke length

So when we're talking about a cylinder, work = pressure * piston area * stroke length. And because the length of the stroke and the area of the piston are fixed, the only way to maximize work is by increasing pressure.

The timing of the spark is important, and the timing can either be advanced or retarded depending on conditions.

The time that the fuel takes to burn is roughly constant. But the speed of the pistons increases as the engine speed increases. This means that the faster the engine goes, the earlier the spark has to occur. This is calledspark advance: The faster the engine speed, the more advance is required.

Other goals, like minimizing emissions, take priority when maximum power is not required. For instance, by retarding the spark timing (moving the spark closer to the top of the compression stroke), maximum cylinder pressures and temperatures can be reduced. Lowering temperatures helps reduce the formation of nitrogen oxides (NOx), which are a regulated pollutant. Retarding the timing may also eliminate knocking; some cars that have knock sensors will do this automatically.

Next we'll go through the components that make the spark.

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The ABS System

The theory behind anti-lock brakes is simple. A skidding wheel(where the tire contact patch is sliding relative to the road) has less traction than a non-skidding wheel. If you have been stuck on ice, you know that if your wheels are spinning you have no traction. This is because the contact patch is sliding relative to the ic. By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You'll stop faster, and you'll be able to steer while you stop.

There are four main components to an ABS system:

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