by Ian Sharp, independent powertrain and suspension engineer
Ian has worked with Lotus, Chrysler, and other companies.
Resplendent in fiery orange, blue, and silver aluminum, a model of the Hellcat engine sat on its stand as engineers discussed technical details of the Chrysler Hellcat powertrain, including the downstream devices that deliver all this power and torque to the aggressive rear wheels.
To be used in the 2015 Charger and Challenger, it is a nicely executed piece of engineering, especially with the claimed 22 mpg (highway) fuel efficiency. The engine does not have cylinder cutoff, making the 22 mpg figure more impressive.
Something that intrigues me is that the fuel flow is directed to impinge onto the back face of the inlet valve, with almost a continuous flow from its injectors at higher rpm. This is said to be for cooling, but it seems like an inefficient way to do it.
Having all this power on tap must mean that all loose objects, including companions, should be securely mounted (if that is the appropriate phrase) within the confines of the cabin, for fear of being pressed into a wad against the back window. A child bolted into the back seat could be considered to be in training for the G-forces of a mission to Mars.
There are numerous interesting aspects to how the conversion from the standard 6.4 liter SRT “Apache” engine was dealt with. For example, the connecting rod bolts are standard Apache bolts, which one might imagine would require double the tensile strength. By careful design, the Hellcat pistons match the weight of the Apache pistons, keeping the same inertial forces. The bolts are in tension, so their load limiting factor, as the piston reaches the top, is the inertia of the piston and rod.
The block was not quite a standard Apache block, but the apparently small changes made to it remain unknown; I believe they were in the crank lower end region. The Hellcat engine is built on the same line as the 5.7 and 6.4 Hemis, with additional operations and stations for the unique components.
The aluminum-alloy (A356) heads are different, with higher strength and thermal conductivity, with heat, quench, and solution treatment. Heat is also handled with hollow stem inlet valves (which reduce inertia and are especially important at higher revs) and sodium-cooled exhaust valves, with temperature resistance of 800° C (continuous). Valves sizes are 54.3 mm for the intake, 42 mm for the exhaust. Hydraulic roller tappets help lengthen the life of the valvetrain.
The rods have tapered small ends to accommodate more material, widthwise, across the piston wrist pin lands.
Similarly, the crankshaft is machined from the same forging as the base Hemi engine forging, but machined (albeit with reduced stroke), to reduce the fixed compression ratios and thus peak cylinder pressures, as the cylinders are filled efficiently by the supercharger. (The cam has a 14.25mm/278° inlet and 14.0mm/304° exhaust lift; durations were measured at 0.15 mm lift). Variable valve timing has 37° phasing authority. It uses “conventional” Hemi valve and port configurations.
These all very nicely engineered solutions within the original 6.4 liter Hemi engine package, to accommodate the increase in the internal loads and forces, by nestling on top of the engine the forced induction system. It might beg the question of whether a forced induction package was envisioned when the new Hemi was designed in the first place, or were things just by serendipity over-engineered, a happy coincidence? An interesting question that a materials accountant manager on the Apache project might have fortuitously overlooked, or that engineers mischievously contrived to obfuscate (much like this sentence).
The “black key” restricted horsepower for those deemed insufficiently skilled to control all 707 horsepower, with a “mere” 500 hp, is like closing the stable door to only three-quarters open.
Chris Cowland, Chrysler’s Director of Advanced and SRT Powertrain, is a Brit not steeped in the history and lore of the bygone Hemi Charger and Challenger. He was also Chief Engineer and Platform Director for the Chrysler Pentastar V6, an engine rather different from the Hemi in approach. Mr. Cowland started out with 13 years at diesel maker Perkins; then in rapid succession moved to Lotus, Ricardo, AVL, and finally Chrysler (July 2008), where he was charged with developing and launching the Pentastar engine.
Mr. Cowland related that the challenges with retaining the accessory drive (which is also the supercharger drive) on the nose of the crankshaft. This required a diamond encrusted washer — not one you could pass off to your intended, but one that is faced with industrial diamonds — to increase the coefficient of friction, and thus the torque transmission capability of the belt drive for the new supercharger, with the available bolt size’s clamping torque. This is a non-tapered drive pulley and crankshaft nose, for the flat faced shoulder drive of the crankshaft and pulley.
The twin spark plus cylinder design has a good deal to do with the reduced emissions, allowing a more complete burning of the fuel and oxygen. This to me has to be a prime example of how CAD (and some ingenuity) is able to help reach the admirable power vs fuel efficiency numbers.
Power is a function of how much air you can get through the engine, and the IHI charge-cooled supercharger bolted on top does this with aplomb. A turbocharged system almost does this for free, albeit with some small losses to back pressure constrictions, while the supercharger costs around 80 hp to pump air into the cylinders at peak pressure [that’s an estimate based on the fact that the belt can drive up to 80 bhp. Most likely, the supercharger requires somewhat less power.]
Current turbocharged engines are now rather civilized, so it may be puzzling as to why they chose the supercharger route, rather than dual turbochargers; the answer is torque, according to a Chrysler spokesman. The Hellcat has over 400 lb-ft of torque from just off idle, one key to having customers feel the power — while horsepower builds in an almost perfectly straight line, torque is high throughout the engine’s operating range for neck-snapping responses [see our Challenger Hellcat drive].
The forced air system has an integral air-to-fluid charged-air cooling circuit, with its own cooling circuit including an electric coolant pump. IHI supplied the supercharger and integral induction air coolers, whilst Chrysler SRT was responsible for the external cooling module and electric fluid pump.
The IHI unit is warranted for the life of the engine, with sealed and independently lubricated bearings. The twin-screw setup, with conformable rotor coatings, can reach 75.5% peak efficiency, and has a maximum boost pressure of 11.6 psi and peak speed of 14,600 rpm.
The complete engine went through a series of OEM highly abusive thermal shock testing dyno durability test, by rapidly bringing the engine up to temp then introducing cold coolant as quickly as possible over a number of cycles to stress the gaskets and metallurgical stability.
One thing that caught my eye was the valve retaining cap and collets, which seat the cap to the upper valve stem; the faces of the collets were pressing hard against one another, which in my experience in racing could prevent the collets from seating firmly on the upper valve stem, especially at higher revs, where the collets can become unseated due to the side pressure. In racing, we used to ensure that there was a good clearance of at least 0.050” (1.25mm) to allow for thermal expansion and build variations. Engine tuners may want to remove 0.020 or so from each collet end face to allow proper seating on the valve stem and cap and also allow for thermal expansion. (They can sometimes “hammer” themselves into a full seating condition, but become a devil of a job to remove in a rebuild.)
[Mr. Cowland responded, “We use multi groove collets on the Hellcat (and all Hemis) — these collets are designed to butt up against each other within the top retainer and allow the valve to rotate freely . The opening/closing force of the valve is transmitted by the beads of the collets into the grooves of the valves.” — editor.]
In all, this seemed to be an efficient, and I would assume a relatively cost effective, way to produce a lot of horsepower and torque from a limited number of engines that are derived from more prosaic production components, that adhere to spirit of those muscle cars of forgone years.
This engine is a testament to new materials, computer modeling techniques, and electronic software, as well as more accurate mixture control (rather than imprecise devices such as carburetors). It a modern application of old style engineering designs. These new tools in an engineer’s toolbox can give new life to old design principles, coupled to existing known solutions to problems in a high power piston engines.
There is nothing startlingly advanced in the engine. To illustrate this point, we can look at the graphite patches affixed to the piston skirt. This technique was standard practice in racing in the 1970s, when we used a Dow Corning aerosol Molykote™ spray can, masking off the piston lands, and sitting the pistons on the workshop gas heater to bake the spray coating on. This was far from the accurate robotic process that I am sure is the method now, but as Chris Cowland pointed out, nothing in engineering is completely new, particularly in reciprocating engine technology.
One wonders how long Chrysler can travel along the retro design route without fresh, exhilarating engineering that fires the imagination of new Jetson kids intent on taking one of Mr. Musk’s one-way tickets to Mars. Nostalgia is fine in its own place, for example the Goodwood revival, but in a discipline that oft repeats the mantra of continuous improvements, it could be viewed is a bit of a contradiction.
The most startling things are the low emissions and high fuel efficiency figures that go along with this prodigious power. I believe a lot of this can be attributed to the ever growing sophistication with which we can computer model our physical universe. One wonders what could have been made use of when designing such iconic engines as the Cosworth DFV and Rolls Royce Merlin, where one has to believe old fashioned knowledge, and the old standby adage “if it looks right, it is right,” along with a great deal of empirical testing, took place.
This engine is quite a good effort, but every car sold will require an increased number of more fuel efficient cars to offset CAFE. The Hellcat Charger and Challenger are expected to draw a $1,700 gas guzzler tax.
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