31-08-2012, 10:36 AM
Paper Presentation On TURBO ENGINE
turbo engine.doc (Size: 320.5 KB / Downloads: 136)
ABSTRACT
Internal combustion engines are "breathing" engines. That is to say, they draw in air and fuel for energy. This energy is realized as power when the air-fuel mixture is ignited. Afterward, the waste created by the combustion is expelled. All of this is typically accomplished in four strokes of the pistons. The greater emphasis on fuel economy, engine responsiveness, and emissions control has driven the ENGINEERS towards the development of turbochargers that serve distinct commercial diesel applications, along with reduction in NOx emmitions.
The basic principal behind turbocharging is fairly simple, but the real thing behind it is not as simple as it looks. A turbocharger is a very complex piece of machinery. Not only must the components within the turbocharger itself be precisely coordinated, but the turbocharger and the engine it services must also be exactly matched.
INTRODUCTION:
Development in automobiles has reached a systematic and logical extention. Specially automobiles today are recognized with their speed & optimum performance. To have only speed is easy but to have it with optimum performance is a difficult nut to crack.
You've heard the word "turbo" tossed around a lot, especially by performance car enthusiasts. But all you know is that it means an engine has more "oomph" to it than normal. But what exactly is going on underneath that hood? Let's open it up and take a look.
TURBO ENGINES:
The underlying basic truth about engine performance is that power output is directly tied to the total amount of fuel that can be burned in the engine. However, it takes air to support the combustion of fuel to create usable power, so increasing power begins with increasing airflow. There are many ways to increase total engine airflow, such as simply building a bigger engine. The real trick is to design an engine system that provides the desired engine airflow and power upon demand — without doing work necessary to pump that extra air into and out of the engine (and the fuel that must be mixed with it) when there’s not a demand for it. Ideally, this would be a small engine with huge power potential. Such an ideal design couples power with economy and efficiency — requirements that seem to be contradictory. Fortunately, such a design solution is both possible and practical. The answer to the above dilemma is turbo engines. A turbo can be a simpler, more compact way to add power, especially for an aftermarket accessory. A turbo can significantly boost an engine's horsepower without significantly increasing its weight, which is the huge benefit that makes turbo engines so popular!
HOW A TURBO SYSTEM WORKS:
If we want our small engine to perform like a big engine, or simply make our bigger engine produce more power, our ultimate objective is to draw more air into the cylinder. By installing a turbocharger, the power and performance of an engine can be dramatically increased.So how does a turbocharger get more air into the engine? Let us first look at the schematic below:
The turbocharger is bolted to the exhaust manifold of the engine. The exhaust from the cylinders spins the turbine, which works like a gas turbine engine. The turbine is connected by a shaft to the compressor, which is located between the air filter and the intake manifold. The compressor pressurizes the air going into the pistons.
DESIGN CONSIDERATIONS :
Before we talk about the design tradeoffs, we need to talk about some of the possible problems with turbochargers that the designers must take into account.
Too Much Boost:
With air being pumped into the cylinders under pressure by the turbocharger, and then being further compressed by the piston, there is more danger of knock. Knocking happens because as you compress air, the temperature of the air increases. The temperature may increase enough to ignite the fuel before the spark plug fires. Cars with turbochargers often need to run on higher octane fuel to avoid knock. If the boost pressure is really high, the compression ratio of the engine may have to be reduced to avoid knocking.
Turbo Lag:
One of the main problems with turbochargers is that they do not provide an immediate power boost when you step on the gas. It takes a second for the turbine to get up to speed before boost is produced. This results in a feeling of lag when you step on the gas, and then the car lunges ahead when the turbo gets moving. One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows the turbine and compressor to accelerate quickly, and start providing boost earlier.
Small vs. Large Turbocharger:
One sure way to reduce the inertia of the turbine and compressor is to make the turbocharger smaller. A small turbocharger will provide boost more quickly and at lower engine speeds, but may not be able to provide much boost at higher engine speeds when a really large volume of air is going into the engine. It is also in danger of spinning too quickly at higher engine speeds, when lots of exhaust is passing through the turbine.
A large turbocharger can provide lots of boost at high engine speeds, but may have bad turbo lag because of how long it takes to accelerate its heavier turbine and compressor.
In the next section, we'll take a look at the optional turbo features which will help us overcome these challenges.
OPTIONAL TURBO FEATURES:
Blow-Off (Bypass) Valves :
The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.
Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to atmosphere to relieve the pressure; helping to eliminate the phenomenon of surge.
SUPERIORITY OF TURBOCHARGERS :
As previously mentioned, it takes additional airflow to support the combustion of extra fuel to increase power. To be a little more technical, it takes additional oxygen to support the combustion of more fuel. There are ways to simulate increased airflow without actually doing it, but such tricks have their short comings. For example, we can get that extra oxygen by introducing oxygen-bearing compounds to the air/fuel mixture entering the engine. Lets look at acouple of ways this is commonly done, and why turbocharging is a better solution. One common method of boosting power by increasing the oxygen available for combustion is to inject nitrous oxide, along with extra fuel, into the engine’s intake system. When the nitrous oxide enters the engine’s cylinders, compression heat causes the nitrous oxide to separate into nitrogen gas and oxygen. That oxygen supports the combustion of the extra fuel. A nitrous oxide injection system can offer a substantial increase in power while the nitrous oxide and fuel are being injected. Such systems are initially less expensive than more durable forms of power enhancement such as turbocharging, but there are some big disadvantages. Most notably, the power gain only occurs while the nitrous oxide and extra fuel are being injected, and it takes quite a substantial amount of nitrous oxide to produce significant power gains. This compressed nitrous oxide must be carried aboard the vehicle in a separate high-pressure tank. In most cases, it is impractical to carry more than 60 seconds worth of nitrous oxide aboard the vehicle. This means nitrous oxide injection is not practical for sustained power output. Even when used only occasionally for short spurts, the tank must be frequently refilled at a nitrous oxide supply station, such as a speed shop. This is both inconvenient and an on-going expense. Turbocharging has no on-going expenses.
CONCLUSION:
Today’s auto world is the world of fast and furious automobiles. Attempts for hike in speed and power with improvement in fuel economy and engine response are thus being made. Turbo engines are one the best solutions that offers optimum speed and power with several additional features such as high boost at lower speed, utilization of waste gas energy, high power to weight ratio, environmental friendly.
Thus the turbo engines play an important role in enhancement of engine working parameters.