Audi A3 launching with new 4-cylinder engines; cylinder-on-demand, supplementary indirect injection, start-stop and modular efficiency technologies
6 May 2012
|1.4 TFSI with cylinder on demand technology. Click to enlarge.|
When the new A3 is launched (earlier post), Audi will be offering one TDI and two TFSI engines, with a 1.4L TFSI with cylinder-on-demand to follow. The initial three four-cylinder engines all make their appearance as new developments. Their engine displacements range from 1.4 to 2.0 liters and their power from 90 kW (122 hp) to 132 kW (180 hp).
Also playing a large role are technologies from the Audi modular efficiency platform. The start-stop system utilizes a new glass mat battery and reduces fuel consumption by up to 0.3 liters (0.11 US gallons) per 100 km (62 miles).The thermal management system ensures that the engine comes up to its operating temperature rapidly after a cold start—this shortens the phase of elevated friction losses due to viscous oil, and the car’s interior warms up faster as well.
|Weight reduction in the Audi A3. Click to enlarge.|
All engines have the same mounting position and orientation in the new A3—this is standardized in the modular transverse platform. In the previous model, some of the gasoline engines were mounted with a forward tilt of 10 degrees, and their exhaust side faced the radiator grille, while the exhaust side of the TDI engine faced the bulkhead. In the new A3, all gasoline engines have their exhaust side facing the rear, and—like the diesel engines—they are also tilted 12 degrees towards the rear.
This mounting arrangement and the compact layout of the new gasoline engines shorten the installation length in front by about 50 mm (1.97 inches) compared to the previous model. This lets Audi developers shift the front axle forward by 40 mm (1.57 inches). The front overhangs were shortened by the same dimension, which benefits crash behavior, styling and the distribution of axle loads.
2.0 TDI. The 2.0 TDI with its 1,968 cc engine displacement (bore x stroke: 81.0 x 95.5 mm) is an entirely new engineering development that was systematically designed for low efficiency losses. The toothed belts for the camshafts and ancillary components run smoothly and quietly. The two balancer shafts, relocated upward from the oil pan to the crankcase, have low-friction bearings; they are lubricated by oil vapors.
Elaborate needle bearings are used for the drive wheels of the camshafts as well. The shafts are pressed into a separate bearing frame—the new valve train module exhibits high rigidity and low weight. In contrast to the previous engine, the cylinder head has a “rotated valve star” layout; each of the two camshafts actuates one intake valve and one exhaust valve per cylinder.
At the pistons, reduced stress on the rings results in smooth running; in manufacturing the engine, a honing process in fine machining of the cylinder liners guarantees high precision. The oil pump, with its two-stage control, is designed for energy efficiency. The actively controlled thermal management system enables friction-optimized hot running: coolant circulation in the cylinder block can be deactivated during the warm-up phase via a switchable coolant pump. Coolant circulation above the cylinder head is then controlled by an electric pump to meet the needs of interior heating and to recover energy from exhaust gas recirculation.
The common rail fuel injection system builds a system pressure of up to 1,800 bar; the fuel is atomized by eight-hole nozzles. The turbocharger was redesigned for the use of low-pressure exhaust gas recirculation. The water cooled intercooler is integrated in the induction pipe—this type of construction leads to short gas paths, high control quality and very good efficiencies. The new low-pressure exhaust gas recirculation system is also very compact and is configured for low flow losses; the emissions control system was located near the engine and is already prepped for the future Euro 6 emissions standard.
The 2.0 TDI in the new Audi A3 produces 110 kW (150 hp); from 1,750 to 3,000 rpm, it transfers 320 N·m (236 lb-ft) torque to the crankshaft. Acceleration from zero to 100 km/h (62 mph) takes 8.6 seconds; acceleration tops out at 216 km/h (134 mph). The combined fuel consumption of the new two-liter diesel is 4.1 liters of fuel per 100 km (57.4 mpg US), which is equivalent to CO2 emissions of 106 grams per km (57.37 g/mile) – about 7% less than in the previous model with 103 kW (140 hp). The new Audi A3 can travel up to 1,200 km (746 miles) on one 50 liter (13.2 US gallons) tank of fuel.
1.4 TFSI. The 1.4 TFSI with 1,395 cc engine displacement (bore x stroke: 74.5 x 80.0 mm) is an engine that is new from the ground up. Its crankcase is made of die cast aluminum instead of the grey cast iron of the previous engine, and it only weighs 18 kg (40 lb) instead of 33 kg (84 lb). Other changes, e.g. to the crankshaft and the connecting rods, contribute towards the new 1.4 TSI weighing only 107 kg (235.89 lb). The 21 kg (46 lb) reduction also yields significant benefits in terms of axle load distribution.
|Aluminium cylinder head with integrated exhaust manifold. Click to enlarge.|
Another new approach is the integration of the exhaust manifold in the cylinder head. It quickly brings the coolant to temperature after a cold start; only then does a thermostat in the new coolant pump module enable cooling of the crankcase. At full load the cooling jacket reduces the temperature of the exhaust gases. Specific tuning of the engine for this behavior achieves substantial improvements in fuel economy—especially with driving profiles, according to Audi.
The aluminum pistons were redesigned; the nearly flat design of the piston heads is precisely tuned to the intake ports that were also redesigned. The fuel injection system operates at pressures of up to 200 bar, and its five-hole injectors can deliver up to three injections per operating cycle.
The turbocharger shed 1.8 kg (3.97 lb) of weight compared to the previous engine. Its newly developed electric wastegate adjuster operates very quickly and precisely, which significantly improves engine responsiveness. The intercooler, which is also integrated in the induction pipe, accelerates the buildup of charge pressure in a way similar to the 2.0 TDI.
In the 1.4 TFSI, friction losses were reduced by up to 20% compared to the previous engine. Improvements were made to the piston rings and their play when mounted in the grey cast iron cylinder sleeves, their reduced diameter at the crankshaft main bearing, lighter-weight valves and toothed belts for the timing and ancillary drives that have been designed to last the life of the engine. The pressure-controlled oil pump also contributes to engine efficiency.
Like the 2.0 TDI, the 1.4 TFSI has a valve train module with low-friction camshaft bearings, with the difference that the shafts are integrated directly in the valve cover. The intake camshaft can be adjusted over 50 degrees of crank angle.
The 1.4 TFSI produces 90 kW (122 hp), and its maximum torque of 200 N·m (148 lb-ft) is available from a low 1,400 rpm. It accelerates the new Audi A3 to a highway speed of 100 km/h (62 mph) in 9.3 seconds and takes it to a top speed of 203 km/h (126 mph). Its combined fuel consumption is 5.2 liters per 100 km (45 mpg US), equivalent to 120 grams CO2 per km (193 g/mile). Compared to the previous model, this represents an improvement of about nine percent.
1.4 TFSI with Cylinder on Demand technology. A second 1.4 TFSI will be added to the engine lineup bringing power and efficiency to a new level. The four cylinder produces 103 kW (140 hp) and offers 250 N·m (184 lb-ft) torque from 1,500 to 3,500 rpm.
The strong 1.4 TFSI utilizes the new “cylinder on demand” (COD) technology, which debuted in a similar form in Audi’s large S models. This is an advanced development of the Audi valvelift system that varies the stroke of the valves on the second and third cylinders of the four-cylinder engine and stops them at low and moderate engine load in coastdown. The system is enabled for engine speeds between 1,400 and 4,000 rpm and torques up to 100 N·m (74 lb-ft).
The valve adjustment is made by sleeves or cam units, each of which has two different cam profiles. When they are are shifted 6.25 mm (0.25 inch) on the camshafts by electromagnetically actuated pins, the zero stroke cam profiles rotate over the exhaust and intake valves. They do not actuate the valves, and the valve springs remain closed; at the same time, fuel injection is deactivated. In the active cylinders 1 and 4, on the other hand, efficiency rises, because their operating points are shifted to higher loads.
When the driver of the A3 pushes the pedal for fast acceleration, the cam units retract, and the shut-down cylinders are reactivated. The entire switchover operation takes 13 to 36 milliseconds, depending on engine speed; switchovers are smoothed by interventions in fuel injection, ignition timing and the throttle valve.
The two-cylinder status is displayed to the driver in the driver information system, when the driver calls up the momentary fuel economy display. Otherwise, the driver would hardly notice the change—even with a 360 degree firing angle, the balanced 1.4 TFSI still runs very quietly and with little vibration; this is also the result of modifications made to the engine mounts, dual-mass flywheel and exhaust system.
The system utilizes information from the accelerator pedal sensor. If the system detects a non-uniform driving profile—such as while driving through a traffic roundabout or in sporty driving on a country highway—cylinder shutoff is disabled; in such cases the deactivation would generally be short-lived and not save any fuel. In third gear, two-cylinder operation can begin at about 30 km/h (18.6 mph), in fifth and sixth gears it ends at 130 km/h (81 mph).
1.8 TFSI and indirect injection. The 1.8 TFSI is the most powerful engine in the new A3 at market launch. Its torque curve is like that of a TDI—its maximum torque of 250 N·m (184 lb-ft) is already available at 1,250 rpm, and it remains constant up to 5,000 rpm. Its power is 132 kW (180 hp).
A major innovation in the 1.8 TFSI is the addition of indirect injection. Complementing FSI direct fuel injection, it is active at part load; it injects the fuel at the end of the induction pipe in the vicinity of the tumble flaps, where it is intensively tumbled with the air. The resulting improved mixture formation boosts fuel economy and reduces particulate emissions. Direct FSI fuel injection, with its up to 200 bar pressure, comes into play in the starting phase and at higher loads.
Audi engineers have discovered new degrees of freedom in filling the combustion chambers. The intake and exhaust camshafts are adjustable; on the exhaust side, the Audi valvelift system also varies the strokes of the valves to further minimize charge changing losses.
The thermal management system is controlled by two rotary valves, combined in a module and driven by an electric motor via a worm gear. They ensure that the engine oil is rapidly heated, and they regulate coolant temperature between 85 and 107 degrees Celsius, depending on the driving situation. At all loads and engine speeds they strike an optimal balance between minimal friction and high thermodynamic efficiency.
As in the 1.4 TFSI, the exhaust manifold of the 1.8 TFSI is integrated in the cylinder head where coolant is circulating; this solution reduces the temperature of the exhaust gases, which improves fuel economy at full load. The turbocharger is also a new development; it builds up its high relative charge pressure of up to 0.8 bar very dynamically. Its main features are an electric wastegate adjuster, which regulates the charge pressure rapidly and precisely, and a turbine wheel made of a new alloy that can handle exhaust gases at temperatures of up to 980 degrees Celsius.
Despite all of its new technologies, the 1.8 TFSI only weighs a little over 140 kg (308.65 lb)—a value that is attained by such measures as reducing wall thickness in the grey cast iron crankcase. The walls are only about three millimeters (0.12 inch) thick, saving about 2.4 kg (5.29 lb). The pistons consist of a new high-strength alloy. The oil pan consists of a lightweight plastic, and many screws in the engine are aluminum.
Internal friction is low as well, due to a new type of coating for the piston skirts, low-friction bearings for the two balancer shafts and a reduced-diameter main bearing for the 1.6 kg (3.53 lb) lighter crankshaft that only requires four counterweights. The regulated oil pump consumes little energy, and a highly precise electrically powered cooling system cools the piston heads with an oil spray as necessary.
The 1.8 TFSI propels the Audi A3 from zero to 100 km/h in 7.2 seconds, and it ends its acceleration at a top speed of 232 km/h (144\ mph). Its combined fuel consumption is 5.6 liters of fuel per 100 km (42.00 mpg US)—equivalent to CO2 emissions of 130 grams per km (209 g/mile). Compared to the previous engine with 118 kW (160 hp), fuel economy was improved by about 13 percent.
Future engines. Audi will be extending its engine lineup for the new A3 step by step. Later this year, a fuel efficient 1.6 TDI with 77 kW (105 hp) will appear, which offers a combined fuel consumption of 3.8 liters of diesel per 100 km (62 US mpg), equivalent to 99 grams CO2 per km (159 g/mile). Vehicle options are unlimited for this highly efficient A3 version.
Furthermore, the future engine lineup will include a more powerful engine for an S model and other versions, including one with an electric drive. Another engine designed for sustainable future mobility is the 1.4 TCNG. The 1.4 liter spark-ignition engine with 81 kW (110 hp) is powered by e-gas, a new, renewably generated fuel from Audi. (Earlier post.) A well-to-wheel analysis shows that its CO2 emissions per kilometer are less than 30 grams (48.3 g/mile).
Seven-speed S tronic and quattro all-wheel drive. At the launch of the new A3, Audi is pairing the 1.4 TFSI and 2.0 TDI with six-speed manual transmissions. The seven-speed S tronic is standard equipment in the 1.8 TFSI; the dual-clutch transmission adds a freewheeling function in conjunction with Audi drive select.
The seven-speed S tronic comprises two subassemblies with a dry clutch; two multiplate clutches serve the gears. The large outer clutch K1 transfers engine torque via a solid shaft to the toothed wheels of gears 1, 3, 5 and 7. Rotating around the solid shaft is a hollow shaft. It connects to the smaller clutch K2, which is integrated internally to its larger counterpart; its output is to the toothed wheels of gears 2, 4 and 6 and reverse gear.
The two sub-transmissions are permanently active, but only one of them is connected to the engine at any given time. If the driver is accelerating in third gear, for example, the second sub-transmission is already engaging fourth gear. The shifting process is performed by switching over the clutches. This takes just a few hundredths of a second and happens without any noticeable interruption in propulsive power.
The seven-speed S tronic, which is about 37 cm (14.57 inches) long, is very compact and only weighs about 70 kg (154.32 lb). Its two clutches do not require any oil cooling. This design aspect already increases efficiency. The clutch and gear engaging unit are supplied with oil from a regulated electric oil pump.
The manual transmission and the seven-speed S tronic both transfer forces primarily to the front wheels; a quattro permanent all-wheel drive will make its appearance later in 2012. Its centerpiece is a further developed, even more dynamically operating hydraulic multiplate clutch; if necessary, it can rapidly redistribute forces between the front and rear axles.
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