Development of 1.2L gasoline turbocharged MPFI engine for passenger car application.
In the emerging technology trend, there is continuous demand for increase in engine performance in terms of power & torque while providing competitive fuel efficiency. Understanding and fulfillment of complex customer requirements with affordable technology is extremely challenging. In order to meet potential conflicting needs and offer 'fun to drive' experience to customers, Tata Motors has developed first in segment turbocharged gasoline MPFI engine. Further in order to create market differentiator, multi drive modes were introduced as segment first feature. The boosted compact 1200 cc engine while developing 90 Ps power, delivers 140 N-m torque over a wide range of 1500-4000 rpm, best suited for Indian drive conditions. This performance boost is nearly 40% over and above performance of comparable NA engine without any compromise on vehicle level fuel efficiency. This paper explain the extensive development activity carried out in comprehensive manner providing good insight into technologies involved in major engine aggregates and brief on multi drive mode system. Also the engine performance results are presented towards the end of paper.
CITATION: Inamdar, S., Ravisankar, M., Panwar, A., Sridhar, S. et al., "Development of 1.2L Gasoline Turbocharged MPFI Engine for Passenger Car Application," SAE Int. J. Engines 10(1):2017,
Tata Motors developed a four cylinder MPFI gasoline turbocharged engine with multi-drive mode for the compact car segment. The big efforts have been put into developing turbocharged engine with pleasant drivability as well as for competitive fuel economy. Consequently, with low and medium torque demand in urban driving condition, the turbocharged engine works in a more efficient speed/load region while with higher torque demand boosting helps to obtain the required performance. This new engine from Tata Motors represents a milestone in the development of gasoline engines. Its performance far surpasses that of conventional NA engines with the same capacity while at the same time offering competitive fuel economy. This paper presents the development aims and technical features of this new engine. Deep investigations were carried out during initial design and development phase by simulating the whole thermodynamic cycle, valve train simulation, engine cooling and lubrication circuit flow simulations. The effect of addition of turbocharging on different engine subsystem such as intake, exhaust, crank-train, valve train, lubrication, cooling system and corresponding challenges during development are explained in this paper. Developed 1200 cc turbocharged engine shows good characteristics in terms of performance, low-end torque, fuel economy and transient response.
DESIGN CONCEPT AND TARGET
Over the past few years Tata Motors has developed a four-cylinder gasoline turbocharged for the compact car segment. This engine currently features eight valve four cylinder with 1.2L displacement and is the first turbo charged version of gasoline engine family with multidrive mode and giving best in class performance. Major components of base engine and auxiliaries were newly developed for this application. In the following sections, detailed development of the base engine and auxiliaries are described and the performance results are presented. Table 1 lists the basic specifications of the engine and Figure 1 shows the appearance of the engine.
The main development targets are listed below:
* Performance boost of about 40% over and above comparable NA engine without any compromise on vehicle level fuel efficiency.
* Deriving multiple driving modes in engine
* Improved NVH performance
* Compact construction
* Better under hood thermal management.
* BS-IV compliant
* Leverage existing engine family manufacturing facilities to limit investment for new engine development
CYLINDER HEAD AND COMBUSTION CHAMBER
The cylinder head is an all new design with two valves per cylinder configuration and with flow tuned ports & combustion chamber to extract maximum from the boosting and for good flow characteristics. The thermal inertia is also improved by optimizing the water jacket volume by 30 % for improving engine warm up rate and lower fuel consumption. The thermostat is integrated into the head for better control and to reduce the number of components. Combustion chamber in cylinder head is of hemispherical shape which proved to be highly efficient combustion chamber with minimal heat loss to the cylinder head and accommodating two large valves and these large valves are necessarily heavier than those in a multi-valve engine of similar valve area, as well as generally requiring more valve lift. The intake and exhaust valves lies on opposite sides of the chamber and necessitate a cross-flow head design. The configuration for the hemispherical chamber is to have the spark plug at the almost top/center of the dome with the valves on both sides (left & right) of the plug. There are no corners or hidden pockets so the entire air/fuel mixture is ignited. The spark plug being in the center of the sphere along with the shape of the chamber allows the explosion to quickly travel throughout the chamber. A long reach nickel-yttrium spark plug with a superior grade ceramic material selected for turbo engine application. The fuel injector selected are of more no. of orifice (eight nos.) which provides good fuel atomization and is effective in reducing wall wetting. Spark plug and injector position in cylinder head combustion chamber and intake manifold optimized for higher combustion efficiency. Figure 2 represents combustion chamber shape. Combustion chamber volume decided based on final required compression ratio of 9.3.
An innovative piston concept was adopted to take the higher peak firing pressures due the boosting without adding mass to the component. The piston is designed specifically for the higher thermal and mechanical load of the turbo engine with a maximum cylinder pressure of around 85 bar. The asymmetric shape of the piston skirt was optimized to reduce friction and improve the durability. The optimized piston skirt is also treated with graphite coated skirt for scuff resistance and wear rate reduction. For controlling the permitted piston temperature range piston cooling oil nozzles are installed on cylinder block for cooling. This also significantly improves piston pin lubrication. Further, the floating piston pin system assembly consisting of piston, piston pin, bushing and connecting rod were optimized for the higher gas and mass inertia forces. A highly sophisticated process of Physical Vapor Deposition (PVD) is used to apply an ultra-thin layer of chromium nitride (CrN) to the piston's top ring. The process ensures high wear resistance and a low friction coefficient. Friction between the ring and cylinder wall has been further minimized by reducing the both ring (top and second) tangential loads and second ring's width. The smaller mass and special surface treatment of the piston skirt and rings yielded additional savings in fuel consumption. Figure 3 shows piston ring assembly.
Connecting rod as shown in Figure 4 designed in order to withstand maximum cylinder pressure of 85 bar, with adequate fatigue safety margin. Also a press-fit bush in the small end was adopted, in order to allow a floating piston pin. The fracture splitting method used for manufacturing connecting rods that are made of high-carbon steel, which are comparatively easy to fracture, in order to improve the accuracy, at the same time cost effectiveness.
Five bearing crankshaft is optimized and evaluated for high peak firing pressure in terms of stress fatigue factor, minimum oil film thickness and torsional vibrations amplitudes and found within limits. High inertia flywheel and vibration damper designed to reduce angular acceleration at flywheel end and improve NVH performance. Figure 4 & Figure 5 shows connecting road and crankshaft assembly.
Figure 6 shows cylinder block for new engine. This is cast iron block with closed deck construction for high stiffness requirement. The weight and load optimized cylinder block concept used for this application. The design enables to utilize the existing manufacturing facilities for production with minimum changes. Because of its good acoustical characteristics additional secondary measures can be minimized. The cooling circuit within the crankcase and the entire engine was optimized using CFD calculations. A multi-layer steel gasket is used for sealing joint between cylinder head and cylinder block.
VALVE-TRAIN SYSTEM AND CAMSHAFT
Center pivot rocker arm overhead cam type valve train selected for this engine application as it offers low friction with roller follower mode and low engine height. It has also simple design and relatively high stiffness. Higher valve spring preload require for higher gas loads to achieve high force factor which increase cam follower contact stresses. Contact pressure between steel roller rocker follower and cam-lobe is above material limit for gray cast iron so steel cam material is used for camshaft development. Steel forging camshaft material of grade AISI 1070 with induction hardening heat treatment process exclusively developed. Sintered valve seat insert selected for high temperature and high stress application. Valve stem and roller rocker follower contact stresses are within material limits evaluated based on kinematic and dynamic simulation at max engine rpm so for this spherical rocker pad type is used.
ENGINE COOLING SYSTEM
Cooling system of new engine consist of longitudinal and transverse flow in the cylinder head for better heat transfer coefficients at high thermal loading and more uniform temperatures, integration of the water pump in cylinder block, integration of thermostat and the temperature sensor in the thermostat housing, timing-belt drive for the water pump. The water pump conveys cooling water to the inlet side of the cylinder block. The coolant flows into a gallery where it is distributed and flows around the cylinders to the exhaust side. Part of the flow is diverted to cool the turbocharger bearings to improve turbocharger reliability. Water cooling for turbocharger prevented problems associated with turbocharger heat soak after heavy running and rapid shut down. The main flow rises into the cylinder head, passes round the exhaust valves, the spark plug bosses, and finally to the inlet valves. The off-take for the heating system is on the cylinder head outlet other side. Depending how far the thermostat opens the coolant flows either to the main radiator or is short-circuited directly back to the water pump. A classical wax thermostat is installed in integral thermostat housing in cylinder head. Coolant flows into the block at the 1st cylinder bore, flows across adjacent cylinder bores and through metering holes along the full length of the head gasket and longitudinally the length of the cylinder head before exiting the head at 4th cylinders. A view of the complete cylinder block and cylinder head water jackets, are shown in Figure 7. During initial design phase, trials with and without oil cooler were done to evaluate engine performance and observed that engine oil temperature and pressure was within limit without oil cooler also. During the basic planning stage of engine, CFD analysis was carried out to develop this cooling system construction and cooling passage areas. Cooling system layout is as shown in Figure 8.
ENGINE LUBRICATION SYSTEM
Low friction, low weight and reduced package were the focus for the design of the oil circuit and the lubrication system. Due to the additional lubrication points compared to equivalent NA engine for example the piston cooling nozzles and the turbocharger, the turbo engine has a higher oil throughput so high flow rate oil pump was selected. A separate gallery supplies the unfiltered oil for piston cooling without pressure loss for the lubrication system. Low weight metallic oil-filter module and oil pump housing with mounting on cast iron cylinder block are used for weight reduction. In cylinder block oil galleries provided to supply oil to crankshaft main bearings and to cylinder head. From cylinder block main oil galleries oil supply given to turbocharger housing for bearing lubrication and oil return connection given to oil crankcase.
In cylinder head oil hole provided in rocker shaft to supply oil to lubricate rocker arm, all camshaft bearing. Cam-lobe and roller rocker follower contact of this engine are subjected to high contact stresses due to cam laws & high spring pre-load so separate oil hole provided in rocker shaft to lubricate the same. This will improve oil film formation ratio on its rolling surface and make it possible to form an oil film thick to prevent metal to metal contact at rolling contact surface to prevent peeling and wear on the rolling surface of the roller irrespective of the roughness of mating cam surface. Thus life of the cam follower is increased. Figure 9 shows cam follower contact lubrication details.
AIR INTAKE SYSTEM
Top mounted air filter scheme adapted for high engine performance and compact layout. The induction system has fresh air duct (snorkel) of long length from its inlet starting point to the entrance of air cleaner to reduce the total induction system noise. Induction system layout is shown below in Figure 10. Intake snorkel diameter, length and air filter volume decided based on required engine performance and intake orifice noise target. An aluminum pipe is used as the intake manifold. It has been bent and shaped into a three-dimensional form, allowing a lightweight and compact intake manifold with a large diameter and optimized port to be employed for improved low-to-mid speed torque. The manifold runner pipes were developed through engine performance simulation and test.
The engine uses an air cooled intercooler. This means that the charge air flows through a cooler and releases its heat via the aluminum fins. These are cooled by the surrounding air. Once the intake air has passed the turbocharger, it is very hot. It is heated to up to 150[degrees]C mainly by the compression process, but also by the high temperature of the turbocharger. As a result, the air has a lower density and less oxygen will reach the cylinder. Cooling the air to just above the ambient temperature will increase the density and more oxygen is fed to the cylinders. Furthermore, the knocking tendency and the production of nitrogen oxide are reduced.
TURBOCHARGING AND EXHAUST SYSTEM
Front exhaust layout selected for better under hood thermal management. A new generation water-cooled gasoline Turbocharger as shown in Figure 11 used to boost the engine's outputs to a very impressive 40% starting from low speed of 1500rpm. The low inertia wheels provide a faster response for a seamless torque delivery. A heat-resistant compact (25% smaller on weight and size) turbo with an integrated electric surge protection valve improves the response and reduces the noise. It is first time in India that an MPFI gasoline engine having turbocharger with integrated electric valve being used for a passenger car application by an OEM. The electro-pneumatic valve controls the waste gate opening for the boost demand control which enables the smoother and linear torque delivery. A turbocharger recirculating valve keeps a portion of air running through the intake side of the turbocharger when the throttle valve is closed and boost pressure is still present. This keeps the turbocharger impeller from slowing down, reducing turbo lag when the throttle is applied again. Both the boost and intake pressures are used to control the waste-gate of the turbocharger. These pressure signals are supplied to the PWM valve, which then sends a pulse-width modulated signal to the wastegate bypass regulator valve. As a result, valve controls pressure supply to the pneumatic actuator unit which directly acts on the wastegate via a connecting actuator link. This control system regulates the turbine speed and sets the maximum boost pressure.
The exhaust manifold is split type. A divider in the manifold ensures a steady flow of exhaust gases to the turbine. The ports of cylinders no.1 and no. 4 and cylinders no. 2 and no. 3 are separated based on the firing order. The divider also prevents the exhaust gas pressure from expanding into the other cylinders. In terms of technology, a traditional cast iron solution was chosen, with bolted flanges which interface turbine and cylinder head thereby reducing cost by 30 % compared to a stainless steel exhaust manifold. The increased content of silicon and molybdenum with stress relieving process enables the exhaust manifold to resist oxidation under high-temperature conditions and to improve the component durability. The maximum allowed temperature for the system is 930 [degrees]C. The three-layer heat shield is attached to the exhaust manifold and was optimized in vehicle tests for hot air flow and heat protection. The catalyst is put close coupled with the turbine exit connected with an elbow for improved light off performance of catalytic convertor and layout reasons as shown in Figure 12. Lambda regulation is controlled by oxygen sensor in front of the catalyst and monitored by second oxygen sensor directly behind the catalyst.
AUXILIARY DRIVE AND TIMING DRIVE
New auxiliary belt drive system with stretch fit belt construction for both alternator and AC compressor drive have been developed specifically for this engine for durable and cost effective solution. These belts are self-tensioning and do not require the presence of a tensioner to adjust tension during engine life. Figure 13 shows auxiliary belt system layout of engine.
A toothed rubber belt drive system was chosen for camshaft drive for better NVH. A mechanical timing belt tensioner is utilized to set the initial belt tension load. Water pump is driven by the timing belt drive and compact unit mounted direct into a volute formed in front face of cylinder block.
ENGINE CRANKCASE VENTILATION
Unlike conventional NA engines, the crankcase ventilation system for a charged engine is more complex. While there is a constant vaccum in the intake manifold of a NA engine, pressure is above atmospheric in the turbo engine. For this new engine the cylinder head cover integrates the labyrinth oil separator and the pressure-regulating valve. In the oil separator, the oil is separated from the gases and drips back into the oil sump. Gases are sent to the intake air as follows. In boost zone engine operation, the blow-by-gas is introduced before the compressor. For this the secondary line was routed between cylinder head cover to turbo suction side. Another primary line ventilates the crankcase housing into the intake manifold during non-boosted engine operation. With an PCV valve, the line is closed to prevent crankcase over pressurization results in terms of combustion and knock resistance at full load. Depending on whether the pressure is lower in the intake manifold or turbo suction side based on engine load condition (part or full load) the valve will open and allow the gases to pass through. In the intake manifold or turbo suction side, the gases mix with the intake air and are fed to the combustion chamber. Figure 14 shows engine crankcase ventilation system for the application.
ENGINE MANAGEMENT SYSTEM
This turbocharged engine comes with a modern ECU with a new generation micro-controller. The Eco friendly lead free ECU provides a digitally precise control for vehicle performance and emissions. The higher performance micro-controller which manage three set of calibration maps for multi drive modes. With the presence of the 'On board' ambient pressure sensor in the ECU, a more precise control of Turbo and altitude correction achieved by uninterrupted drive pleasures even in remote areas.
'Multi Drive Modes' are one the key innovations introduced by Tata Motors with the new turbo engine. This engine offers the optimum blend of performance, refinement and fuel economy. The unique first in segment multi drive mode technology enables the driver to match the engine response exactly to how he wants to drive. There are three modes available; City, Sport and Eco. The desired mode is selected by a mere flick of a switch on the dash panel even 'on the fly'. Utilizing impressive engine characteristics of the engine, this objective is achieved by calibrating accelerator pedal map differently for each mode by limiting max torque and power and at the same time by tweaking drivability filters intelligently. The cleverly tuned calibration data with the intelligent algorithms inside the ECU 'morphs' the engine deliver accordingly. Figure 15 shows engine performance maps for different driving modes. Figure 16 and Figure 17 shows throttle and normalized torque vs accelerator pedal positions.
A mode gives the sharpest throttle response and quickest acceleration. All 140 N-m of torque from the new engine is available to give a more dynamic drive enabling the driver to exploit the full acceleration and drivability of car. This mode is ideal for weaving around the traffic or simply to have driving fun on sweeping roads outside the city. A Specially calibrated to provide the driving pleasure extracting the maximum from the new engine.
This mode provides the perfect balance between performance and economy. Power delivery is smooth and throttle response is linear leading to agile drivability to easily navigate city traffic conditions. On the highway this mode provides a good balance between overtaking and long range mileage while driving at comfortable cruising speeds.
When the driver wants to be frugal and extract the maximum mileage from his car he can select Eco Mode. In this mode performance and throttle response is reduced to maximize fuel economy but with comfortable drivability. The air conditioning system control is changed to reduce the energy demand while still maintaining a cooling effect instead of shutting down the air conditioning completely. The Close coupled catalytic convertor along with the refined calibration keeps the output at the tail pipe always 'greener'!
ENGINE PERFORMANCE RESULTS
The full load performance curves in WOT operation is shown in Figure 18. Targets were achieved with max. power of 90 Ps and max. torque of 140 N-m more than 40% above equivalent NA engine. This high engine performance with flat torque characteristics was obtained by adopting smaller compressor and turbine wheel turbocharger.
Figure 19 shows brake specific fuel consumption graph in part load condition. It can be observed that engine achieves extremely low fuel consumption of 260 g/kWh at the best point. At 2000 rpm and 2 bar BMEP condition fuel consumption is slightly higher than equivalent NA engine due to lower engine compression ratio. Also in full load condition and at engine rated speed due to restrictions on the turbine inlet gas temperature, the fuel injection quantity must be increased to limit the exhaust temperature and hence fuel consumption is high.
This paper has introduced development aims and technical feature of new 1200 CC gasoline turbocharged engine having best in class performance. For a competitive hatch for meeting customer expectation, boosting methods like turbo charging provides optimum solution. The engine delivers good characteristics in terms of performance, low end torque, fuel economy despite having standard MPFI injection system. This high performance engine became enabler for developing market differentiator feature i.e. multi-drive mode system.
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[3.] Kojima, S., "Development of High-Performance and Low-Emission Gasoline Engine," SAE Technical Paper 2008-01-0608, 2008, doi:10.4271/2008-01-0608.
[4.] Shimizu, M., Yageta, K., Matsui, Y., and Yoshida, T., "Development of New 1.6Liter Four Cylinder Turbocharged Direct Injection Gasoline Engine with Intake and Exhaust Valve Timing Control System," SAE Technical Paper 2011-01-0419, 2011, doi:10.4271/2011-01-0419.
Sameer Inamdar, Sr-Manager, ERC-Engine Design Tata Motors Ltd. Pune email@example.com
The author would like to extend his thanks to all the members of the Engineering research center team and Manufacturing team of Tata Motors who have participated in the design, development and production of this engine. Without their dedication and co-operation, the achievement of this project targets would not have been possible.
MPFI - Multi point fuel injection
CC - Cubic capacity
NVH - Noise vibration and harshness
CFD - Computational fluid dynamics
AISI - American Iron and Steel Institute
TC - TC Turbocharging
ECU - Engine control unit
OEM - Original equipment manufacture
PCV - Pressure control valve
BMEP - Brake mean effective pressure
NA - NA
WOT - Wide open throttle
AC - Air conditioning
Sameer Inamdar, M Ravisankar, Anupam Panwar, S Sridhar, Viswanatha Hosur, and K Chandru
Tata Motors, Ltd.
Table 1. Basic specification of engine Parameter 1200 cc Gasoline Turbo Engine Specification Bore x Stroke(mm) 75.0x67.5 Compression ration 9.3:1 Max power (Ps @rpm) 90@5000 Max torque (N-m @rpm) 140@1500-4000 Cylinder block/Cy. Head Cast iron/Aluminum Turbocharger Water cooled Boost control Waste gate type Fuel injection type
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|Author:||Inamdar, Sameer; Ravisankar, M.; Panwar, Anupam; Sridhar, S.; Hosur, Viswanatha; Chandru, K.|
|Publication:||SAE International Journal of Engines|
|Date:||Feb 1, 2017|
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