Factory turbo engines seem to be making a comeback - after a decade or so where new turbo engines were something of a rarity, more and more manufacturers are again recognising the packaging, fuel economy and mid-range torque benefits of turbocharging. However - and it's easy to forget this when you look at the range of bolt-on turbo kits that are around - when a major manufacturer increases the power of an engine by adding a turbocharger it's anything but a quick and easy process. While the aftermarket might just make a new exhaust manifold - or in good installs, have one cast - and make some management and intake plumbing tweaks, Original Equipment Manufacturers go right through the engine design, modifying or building new parts as necessary to maintain durability and driveability. Not to mention, pass all legals...
In this story we'll take a technical look at the 2.4-litre DaimlerChrysler engine that's been released for use in the US-market PT Cruiser. With a power output of 160kW (215hp) it's no ground-breaker in the top-end stakes, but its amazingly flat torque curve - which peaks at 332Nm at 3600 rpm - gives a better indicator of where OE turbocharging is headed.
The engineers assigned the task of developing the turbo version of the engine had four main objectives. The turbo engine had to:
That last one's pretty important - especially when you see how many parts in fact ended up being changed!
The 2.4-litre, four cylinder engine shares the same basic architecture as its naturally aspirated (NA) brother - a bore of 87.5mm and a stroke of 101mm, giving a swept capacity of 2429cc. It is a DOHC 16-valve design, using a cast iron block and alloy head. Twin balance shafts are used.
The first step was to design new pistons - something deemed necessary since peak combustion pressures are 50 per cent higher than in the NA engine. Further strength was required because the engine was to be able to operate at stoichiometric (14.7:1) air/fuel ratios, even at wide-open throttle! This is required for US-06 US government test procedures. Since turbo engines normally run relatively rich mixtures (eg 12:1) at full throttle, such a requirement meant that the pistons had to be able to withstand much higher than normal in-cylinder temperatures. Finally, a full floating pin design was used in the new design. The resulting pistons are cast - rather than forged - from Mahle 124 eutectic alloy, with their skirts coated in Mahle Grafal to protect against scuffing and improve Noise, Vibration, Harshness (NVH). The new pistons lower the engine's compression from 9.4:1 to 8.1:1.
The pistons used in the NA engine have a 'ski ramp' crown shape - the top of the piston projects upwards in an asymmetric wedge. This shape was retained for the turbo engine - on both it and the NA engine, computational fluid dynamics (CFD) modelling showed improvements to the air/fuel mixture with this shape. These modelled gains were proved on the engine dyno, with the piston crown shape showing improved wide-open throttle spark and idle stability.
The top land (space between the upper ring and the crown of the piston) was set at 4mm - the smaller this distance, the less crevice volume remains where incomplete burns of the air/fuel mix can occur. However a short land also results in the ring being subjected to higher temperatures, and so hard anodizing was added to the top ring groove to prevent the compression ring welding itself to the piston. To help cool the pistons, oil squirters were mounted in the block. These spray onto the underside of the pistons for the full length of their stroke. Interestingly, during testing thermistors were mounted on the pistons so that their real-time temperatures could be measured.
The rings for the turbo engine are also new. A 1.2mm barrel-shaped steel upper compression ring is used; it has a molybdenum face coating to reduce wear. The second ring is wider at 1.5mm. It is made from grey iron and doesn't use chrome plating, as is commonly the case. A conventional 3-piece oil control ring assembly is also used.
In addition to the new pistons, new con-rods were developed - the NA engine's con-rods couldn't meet durability requirements in the turbo engine application. Previous turbo Chrylser engines have used a two-piece forged steel design, and in this application a 'crackable' (ie the big end is cracked apart during the manufacturing process) C-70 forged steel rod was used. These rods are 78 grams lighter at their big-ends than previous Chrysler Turbo rods, making this end of the rod similar in mass to the NA rods. This allowed common machining of the crankshaft for both Turbo and NA engines. The rod bolts in the turbo engine were upgraded from M9 fasteners to MJ9. The MJ specification reduces the likelihood of stress concentration by providing a more generous thread radius.
The camshaft profiles of the NA engine were retained in the turbo design. However, the intake and exhaust valves were upgraded in material - intake valves are Silchrome-1 and the exhaust valves are Inconel. The main and connecting rod bearings are similar to those used in the NA engine, however the con-rod bearings have additional oil feed holes in them and the thrust bearings have a higher load-carrying capacity, provided by contoured faces.
The use of an intercooler required, for packaging reasons, a change in the intake manifold. The new intake was manufactured from AA319 sand-cast alloy, rather than being made from plastic as is the case in the NA engine. This alteration was not in response to higher underbonnet temperatures; instead the development and manufacturing lead times could be reduced. Aluminium also yielded some NVH advantages over the plastic design.
So with the opportunity to redesign the flow and resonance properties of the turbo engine intake manifold, what changes were made? The main alterations were in the location of the throttle body, the runner length and the plenum volume. Runners 1 and 4 are each 388mm long, while 2 and 3 are 382mm in length. The intake system is said to have good cylinder-to-cylinder distribution, varying by a maximum of about 10 cfm at 0.200 inches valve lift, by 5 cfm at 0.300 inches lift, 7 cfm at 0.400, and about 10 cfm at 0.450 inches lift. (The measurements were taken at 25 inches of water, and the maximum flow was just under 200cfm at 0.400 inches lift.)
With the increased demand of the piston oil squirters and the turbo bearing, a new oil pump was deemed to be necessary. Using a thicker geroter design pump (a displacement increase from 12cc to 15cc per revolution) gave the desired results. In addition, new oil lubrication and coolant plumbing were installed to service the turbo.
A completely new water pump was used. The NA engine's stamped steel 6-blade impeller was replaced with a injection-moulded 7-blade impeller. In addition, the water pump housing was modified to give tighter clearances, improving pump efficiency. The revised design lifted flow by 25 per cent and gave a 48 per cent increase in pressure across the pump at 5000 rpm.
The turbo and NA blocks are largely the same; in fact the changes made for the turbo engines have now been implemented on both NA and turbo blocks. These included oil and water bosses for the turbo supply, a water supply boss for the oil/coolant heat exchanger, and an oil return boss for the turbo. Cast pads were added to the inside of the block to allow for the installation of the piston oil squirters.
However, the greatest changes to the block were the alterations in water jacketing. Two cast ramps were placed adjacent to #1 cylinder and water diverters were located between cylinders 1-2 and 3-4. These changes were aimed at improving cooling between the bores at the deck height. CFD modelling indicated that the changes would increase the flow of coolant in the between-bore passages by a factor of six times.
A new multi-layer steel head gasket is used on the turbo engine. These needed to be designed to cope with higher combustion pressures, higher head and block temperatures, and increased head lift. The new gasket is constructed of three layers of 0.8mm stainless steel and is coated with a thin elastomer.
A reverse-rotation Mitsubishi TD04-16GK turbo was selected for the engine. It uses an integral wastegate and the turbine housing and wastegate passage are integrated into the exhaust manifold. The turbo and manifold assembly has a mass of 11.6kg. The reverse rotation design was selected to allow for better packaging in the tight under-bonnet space that was available. Boost pressure rises as high as 14 psi and an air/air intercooler is used to reduce intake temperatures.
The integration of the turbine housing into the cast exhaust manifold was done so that a compact assembly could be maintained and also so that the cat converter would be more quickly heated - the thermal inertia of the overall system is reduced. The exhaust manifold uses a dual-plane approach with paired runners, a design that underwent extensive CFD and flowbench development. The manifold is cast from Ni-resist D5S cast iron, a material selected for its durability at high exhaust gas temperatures.
The engine management system is DaimlerChrysler's Next Generation Controller (NGC) system. Its control strategies are model-based, rather than relying on pre-programmed maps of data. Intake airflow is determined using a MAP sensor - the so-called speed-density system. Boost control is also based on a model, this time a torque request system. The driver's throttle input is matched by a given torque outcome, determined primarily by the boost that is provided. This torque outcome will be the same irrespective of temperature, barometric pressure and so on. The same model can be used to reduce torque if the engine durability is likely to suffer. For example, spark fuel and boost can be adjusted on the basis of information received from a block-mounted knock sensor.
Three new solenoids were added to the turbo engine management control. One is used to switch the Throttle Inlet Pressure sensor to measuring barometric pressure during steady state engine operating conditions, one activates a blow-off valve as required, and the other controls the wastegate opening and so the boost level.
The 2.4 litre engine develops more than 90 per cent of its peak torque from 2300 to 5000 rpm. Equipped with a 4-speed automatic trans, the 1500kg PT Cruiser fitted with this engine reaches 60 mph (97 km/h) in 7.2 seconds - around 3 seconds better than the NA car - and does the standing quarter mile in 15.7 seconds.
However, more than anything else, the turbo version of the PT Cruiser engine shows the lengths to which manufacturers go to when upgrading and altering the internals on OE turbo engines. If you have a choice between swapping-in a factory turbo engine or bolting a turbo to your NA engine, the former certainly has a better engineering outcome.