Variable Geometry Turbochargers...

Introduction

       An alternative to the fixed geometry turbine is the variable geometry turbine. The benefits of variable geometry turbines over wastegated turbines include:

• no throttling loss of the wastegate valve;
• higher air–fuel ratio and higher peak torque at low engine speeds;
• improved vehicle accelerations without the need to resort to turbines with high pumping loss at high engine speeds;
• potential for lower engine ΔP (the difference between exhaust manifold and intake manifold pressures);
• control over engine ΔP that can be used to drive EGR flow in diesel engines with High Pressure Loop (HPL) EGR systems;
• a better ability to cover a wider region of low BSFC in the engine speed–load domain;
• ability to provide engine braking;
• ability to raise exhaust temperature for aftertreatment system management.

The idea of using a variable geometry turbine in a turbocharger dates back at least to the 1950s . Since that time, a number of different designs have appeared. Two of the more common ones are the pivoting vane and moving wall types, Figure 1 . Others include the variable area type, variable flow type and the sliding ring designs. These designs will be discussed in more detail in the following section.

Figure 2. Comparison of Fixed Geometry (BorgWarner KP39) and Variable Geometry (BorgWarner BV40) Mass Flow vs. Pressure Ratio

The peak efficiency of a variable geometry turbine occurs at about 60% nozzle opening. It is usually comparable to or a few percent lower than that for a fixed geometry turbine. However, efficiency drops off rather quickly as nozzle opening is reduced or increased from a mid-vane opening position, Figure 3 .

Figure 3. Effect of variable geometry turbine nozzle opening and blade speed ratio on turbine efficiency

Audi to turn on OLED taillights at the Frankfurt Motor Show

Audi will reveal a concept car with OLED taillights in Frankfurt (Credit: Audi)

German luxury auto marques have been waging an all-out technology war, working overtime to beat each other to the punch in areas like autonomous driving and infotainment. A smaller but still notable theater of the war pits the brands against each other in developing high-tech lighting solutions. After mastering LED technology, BMWand Audi raced last year to release the world's first production car with laser lighting, and now they're set to battle it out in OLEDs. Audi will launch the latest offensive in September, when it reveals its first OLED Matrix-equipped concept car.

Audi fancies itself the "leading brand in automotive lighting technology" and it sees a real future in OLED technology. BMW surely would disagree with the first part of that statement, but it seems to agree with the latter, having showcased the OLED taillight-equipped Vision Future Luxury concept at last year's Beijing Motor Show.

Audi has been working on OLED taillights for years. It's now ready to follow up the artistic OLED Swarm taillight with a more production-ready taillight design it calls Matrix OLED. The Frankfurt show car will be the first concept car to wear the new lights.

The automaker explains that its OLED technology offers several advantages over other light technologies, including LED. Its Matrix OLED taillights provide flat, homogenous light with continuously variable dimming. They require less cooling and don't need reflectors or light guides, ensuring lightweight, efficient performance. The OLED lights can also be organized into different color and brightness subgroups, opening up new lighting design possibilities.

Audi promises fast switchover times and clean, precise borders between lights of different groups.

Audi is still perfecting its OLED technology and plans to replace today's thin glass sheet OLEDs with flexible plastic films that can be shaped into new, interesting three-dimensional light structures. Increases in light density will allow for OLED brake lights and turn signals.

The company has not revealed what car will wear the OLED lights, but they are expected to debut on an electric crossover concept in Frankfurt previewing a future model scheduled for production in early 2018. An electric concept car seems like the perfect show vehicle for highlighting an all-new, cutting-edge lighting technology, though Audi could save the lights for a separate show car.

Gizmag will be on the floor of the IAA Cars 2015 Frankfurt show starting on September 15. We'll have more details about Audi's concept car(s) and other new debuts and technologies.

Source: Audi

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CRDI (Common Rail Direct Injection)

     CRDi stands for Common Rail Direct Injection meaning, direct injection of the fuel into the cylinders of a diesel engine via a single, common line, called the common rail which is connected to all the fuel injectors.

     Whereas ordinary diesel direct fuel-injection systems have to build up pressure anew for each and every injection cycle, the new common rail (line) engines maintain constant pressure regardless of the injection sequence. This pressure then remains permanently available throughout the fuel line. The engine's electronic timing regulates injection pressure according to engine speed and load. The electronic control unit (ECU) modifies injection pressure precisely and as needed, based on data obtained from sensors on the cam and crankshafts. In other words, compression and injection occur independently of each other. This technique allows fuel to be injected as needed, saving fuel and lowering emissions.

     More accurately measured and timed mixture spray in the combustion chamber significantly reducing unburned fuel gives CRDi the potential to meet future emission guidelines such as Euro V. CRDi engines are now being used in almost all Mercedes-Benz, Toyota, Hyundai, Ford and many other diesel automobiles.

History

     The common rail system prototype was developed in the late 1960s by Robert Huber of Switzerland and the technology further developed by Dr. Marco Ganser at the Swiss Federal Institute of Technology in Zurich, later of Ganser-Hydromag AG (est.1995) in Oberägeri. The first successful usage in a production vehicle began in Japan by the mid-1990s. Modern common rail systems, whilst working on the same principle, are governed by an engine control unit (ECU) which opens each injector electronically rather than mechanically. This was extensively prototyped in the 1990s with collaboration between Magneti Marelli, Centro Ricerche Fiat and Elasis. The first passenger car that used the common rail system was the 1997 model Alfa Romeo 156 2.4 JTD, and later on that same year Mercedes-Benz C 220 CDI.

     Common rail engines have been used in marine and locomotive applications for some time. The Cooper-Bessemer GN-8 (circa 1942) is an example of a hydraulically operated common rail diesel engine, also known as a modified common rail. Vickers used common rail systems in submarine engines circa 1916. Early engines had a pair of timing cams, one for ahead running and one for astern. Later engines had two injectors per cylinder, and the final series of constant-pressure turbocharged engines were fitted with four injectors per cylinder. This system was used for the injection of both diesel oil and heavy fuel oil (600cSt heated to a temperature of approximately 130 °C). The common rail system is suitable for all types of road cars with diesel engines, ranging from city cars such as the Fiat Nuova Panda to executive cars such as the Audi A6.

Operating Principle

     Solenoid or piezoelectric valves make possible fine electronic control over the fuel injection time and quantity, and the higher pressure that the common rail technology makes available provides better fuel atomisation. In order to lower engine noise, the engine's electronic control unit can inject a small amount of diesel just before the main injection event ("pilot" injection), thus reducing its explosiveness and vibration, as well as optimizing injection timing and quantity for variations in fuel quality, cold starting and so on. Some advanced common rail fuel systems perform as many as five injections per stroke.

     Common rail engines require very short (< 10 second) or no heating-up time at all , dependent on ambient temperature, and produce lower engine noise and emissions than older systems. Diesel engines have historically used various forms of fuel injection. Two common types include the unit injection system and the distributor/inline pump systems (See diesel engine and unit injector for more information). While these older systems provided accurate fuel quantity and injection timing control, they were limited by several factors:

• They were cam driven, and injection pressure was proportional to engine speed. This typically meant that the highest injection pressure could only be achieved at the highest engine speed and the maximum achievable injection pressure decreased as engine speed decreased. This relationship is true with all pumps, even those used on common rail systems; with the unit or distributor systems, however, the injection pressure is tied to the instantaneous pressure of a single pumping event with no accumulator, and thus the relationship is more prominent and troublesome.


• They were limited in the number and timing of injection events that could be commanded during a single combustion event. While multiple injection events are possible with these older systems, it is much more difficult and costly to achieve.


• For the typical distributor/inline system, the start of injection occurred at a pre-determined pressure (often referred to as: pop pressure) and ended at a pre-determined pressure. This characteristic resulted from "dummy" injectors in the cylinder head which opened and closed at pressures determined by the spring preload applied to the plunger in the injector. Once the pressure in the injector reached a pre-determined level, the plunger would lift and injection would start.

     In common rail systems, a high-pressure pump stores a reservoir of fuel at high pressure — up to and above 2,000 bars (psi). The term "common rail" refers to the fact that all of the fuel injectors are supplied by a common fuel rail which is nothing more than a pressure accumulator where the fuel is stored at high pressure. This accumulator supplies multiple fuel injectors with high-pressure fuel. This simplifies the purpose of the high-pressure pump in that it only has to maintain a commanded pressure at a target (either mechanically or electronically controlled). The fuel injectors are typically ECU-controlled. When the fuel injectors are electrically activated, a hydraulic valve (consisting of a nozzle and plunger) is mechanically or hydraulically opened and fuel is sprayed into the cylinders at the desired pressure. Since the fuel pressure energy is stored remotely and the injectors are electrically actuated, the injection pressure at the start and end of injection is very near the pressure in the accumulator (rail), thus producing a square injection rate. If the accumulator, pump and plumbing are sized properly, the injection pressure and rate will be the same for each of the multiple injection events.

Advantages & Disadvantages

Advantages

     CRDi engines are advantageous in many ways. Cars fitted with this new engine technology are believed to deliver 25% more power and torque than the normal direct injection engine. It also offers superior pick up, lower levels of noise and vibration, higher mileage, lower emissions, lower fuel consumption, and improved performance.

     In India, diesel is cheaper than petrol and this fact adds to the credibility of the common rail direct injection system.

Disadvantages

     Like all good things have a negative side, this engine also have few disadvantages. The key disadvantage of the CRDi engine is that it is costly than the conventional engine. The list also includes high degree of engine maintenance and costly spare parts. Also this technology can’t be employed to ordinary engines.

Applications

     The most common applications of common rail engines are marine and locomotive applications. Also, in the present day they are widely used in a variety of car models ranging from city cars to premium executive cars.

     Some of the Indian car manufacturers who have widely accepted the use of common rail diesel engine in their respective car models are the Hyundai Motors, Maruti Suzuki, Fiat, General Motors, Honda Motors, and the Skoda. In the list of luxury car manufacturers, the Mercedes-Benz and BMW have also adopted this advanced engine technology. All the car manufacturers have given their own unique names to the common CRDi engine system.

     However, most of the car manufacturers have started using the new engine concept and are appreciating the long term benefits of the same. The technology that has revolutionized the diesel engine market is now gaining prominence in the global car industry.

     CRDi technology revolutionized diesel engines and also petrol engines (by introduction of GDI technology).
     By introduction of CRDi a lot of advantages are obtained, some of them are, more power is developed, increased fuel efficiency, reduced noise, more stability, pollutants are reduced, particulates of exhaust are reduced, exhaust gas recirculation is enhanced, precise injection timing is obtained, pilot and post injection increase the combustion quality, more pulverization of fuel is obtained, very high injection pressure can be achieved, the powerful microcomputer make the whole system more perfect, it doubles the torque at lower engine speeds. The main disadvantage is that this technology increase the cost of the engine. Also this technology can’t be employed to ordinary engines.