Machine Design
The LT4: Another Legendary Corvette Engine

The LT4: Another Legendary Corvette Engine

Chevy and GM engineers have developed and manufactured their most powerful production engine ever.

The 2015 Corvette Z06 will be powered by a fuel-injected small-block engine, the 376-in3 (6.2-l) LT4, that generates up to 650 hp and 650 ft-lb of torque. The engine gives the roadster a top speed of over 200 mph and a 0-to-60 time of 2.95 sec. It can also deliver 15 mpg driving in the city and 24 mpg out on the highway, according to the EPA.

(Be sure to also check out some of the legendary Corvettes throughout the years. Click here.)

A look inside the LT4 engine reveals many of the components behind the three technological features used to make the 650-hp engine efficient and powerful: direct fuel injection, Active Fuel Management (or cylinder deactivation), and continuously variable valve timing. (Click on the image for a larger view)

Feeding it fuel

An electric pump in the 2015 Corvette’s fuel tank sends fuel (premium) to the engine at about 400 to 500 KPa (58 to 72 psi). This would be enough gas pressure for a port-fuel-injected (or non-direct injected) engine, but not for the LT4 and its direct fuel injection. Instead, the pre-pressurized gas goes to a compact megapump, in Chevy parlance. It mounts on the back of the engine and a camshaft running through the engine block drives it. A lifter mechanism inside the megapump translates the rotation of the three peaks or lobes on the end of the camshaft into the up-and-down motion of a piston that compresses the gas. It delivers outgoing fuel pressures up to 20 MPa (2,900 psi), according to John Rydzewski, Assistant Chief Engineer on Chevy’s small-block engines.

The pressurized gas travels into a fuel rail that consists of two arms, one for each row of fuel injectors above both cylinder banks. The rail holds gas at 2,900 psi until it is needed by the injectors. The interior of the fuel rail, as well as the access holes to the injectors, were shaped and placed using CFD and computer simulations to eliminate hydraulic pulsations in the fuel. This ensures fuel pressure is equal and consistent at all eight injector locations when each injector fires.

The injectors had to be made larger than those in the previous LT1 engine to increase the amount of fuel needed by the higher airflow. The shape and geometry of the injector nozzles were carefully sculpted to generate a proven spray pattern of precisely sized fuel droplets that burn thoroughly and fast when mixed with the incoming air and spark.

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Supercharging the engine

The forged aluminum piston heads are strong enough to withstand the 10.1 compression ratio. The top is machined with a distinctive cleft that ensures the pressurized fuel spray and air mix burns quickly and thoroughly and generates the maximum force on the piston. Each piston features three rings or seals: a PVD-coated top ring, a chrome-coated compression ring, and a nitride oils ring. The bottom ring also has eight oil drains.

That incoming air, in most cases, gets a 9.71-psi boost from the Eaton R1740 supercharger/intercooler assembly mounted in the valley between the cylinder heads. The 1.7-liter supercharger delivers 37% more horsepower and 40% more torque than the LT1’s supercharger, yet is only one inch taller. It’s also 20 lb lighter than the earlier supercharger. The R1740 was custom built for the LT4. The serpentine belt running off the crankshaft powers it.

It was important the supercharger be as compact as possible so that the finished roadster would meet pedestrian protection regulations enforced mainly in Europe. Those regulations call for some give or compliance in the hood so that if a pedestrian is hit and falls on the hood, they are not falling on the unforgiving surface of the hood backed up by the engine. Instead, space between the hood and engine lets the hood give or dent a little and cushion the pedestrian’s impct.

Inside the supercharger, a pair of four-lobed rotors spins at 20,000 rpm. The rotors each have a 160° helix (or twist) that broadens their effectiveness by letting them push more air, create more torque at lower rpms, and sustain the boost at higher rpms for more horsepower.

The rotors are also shorter and have a smaller diameter than those in the previous LS9 supercharger. This gives them less mass and inertia, making it easier for the engine to spin them. But because so much air goes through the supercharger at such high speeds, Eaton and Chevy had to stiffen the housing with ribs and add dampeners to reduce resonance effects which can create vibrations and a loud whine.

The fuel subsystem on the LT4 includes the U-shaped fuel rail that holds a reservoir of gasoline at 2,900 psi fed by the megapump and is connected to the eight injectors. Each injector can deliver up to 25 cc/sec (1.5L/min) of fuel into its respective cylinder, according to Chevy engineers.

To make the supercharger/intercooler as efficient as possible, Chevy engineers crafted the intake and output side to be as airtight as possible, and the airflow passageways to be as unrestricted as possible. This includes the pathway from the induction equipment, through the throttle body, and to the supercharger, as well as the discharge port coming out of the rear of the supercharger.

When air exits the supercharger, it travels through a V-shaped window and bends back up and over the intercooler and into the upper portion of the manifold. The design team again used CFD to minimize losses, reduce turbulence and heating, and let the air flow as fast as possible. Results from CFD were used to subtly alter the shape of the window, where it’s located, and how the path is contoured to smoothly take the air where the engineers wanted it to go. The supercharger only comes on when the engine computer determines there is a demand for it, otherwise, it is bypassed. Drivers cannot force it on or off.

If drivers want the best mileage, they can set the transmission in the “Econ” mode (as opposed to the Street or Track modes). When in the Econ mode and there is little demand on the engine, as when cruising at 55 mph on a highway, Active Fuel Management (AFM) kicks in.

Compared to other supercar engines, the LT4 is a veritable fountain of low-end torque, cranking out 457 lb-ft just off idle and 625 lb-ft by only 2,800 rpm. The V-12 in the Ferrari F12 Berlinetta, on the other hand, costs about $320,000 but generates 28% less torque, despite offering 12% more horsepower—and it doesn’t hit its peak torque until 6,000 rpm. The LT4 maintains 90% of its peak torque, or 592 lb-ft, from 2,500 to 5,400 rpm. The LT4 also eclipses the engine in the $150,000 Porsche 911 Turbo S by 90 hp and 134 lb-ft of torque.

AFM closes off a set of hydraulic valves and deactivates the lifters in four of the cylinders. The lifters still go up and down on the camshaft lobes, and the pistons rise and fall inside the cylinders as they remain attached to the crankshaft, says Rydzewski. But the pushrods remain inactive, so no fuel or air is let in, and the exhaust port does not open. The other four cylinders are working normally and receive a bit more gas than if all eight were working, but overall, AFM reduces the amount of fuel burned per mile.

Tougher and lighter

The performance figures for the LT4 engine are an indication of how much stress the engine components must withstand. Some parts are moving incredibly quickly, and temperatures and pressures are high. But Chevy engineers want parts to be light so that it takes little energy from the engine to move them. They must also stay relatively cool so they do not wear or weaken. Here are some of the technological highlights of the engine components:

• Rotocast aluminum cylinder heads. Rotating the mold during casting eliminates porosity in the metal, creating a denser, more accurate head. The heads are then stronger and better able to handle heat than conventionally cast aluminum heads.
• Combustion chambers are larger than those in the LT1: 65.47 cc rather than 59.02 cc.
• Solid titanium intake valves are strong, lightweight, and have exceptional heat resistance.

• Exhaust valves have a sodium center to improve cooling.
• Connecting rods are machined out of forged powder-metal steel, which reduces the amount of reciprocating mass in the engine and lets the engine rev quicker.

• The camshaft has a longer exhaust duration, which holds the exhaust valve open longer to let the greater volume of combustion products escape.
• Lightweight balancer is made of aluminum.

• The crankshaft is forged out of steel and has tungsten balancing inserts and ground collar pins.
• Cast austenitic stainless-steel exhaust manifolds provide more strength at high temperatures than cast-iron manifolds on earlier engines.

• Dry-sump oiling subsystem has a dual-pressure-control oil pump for more efficient cooling and lubrication.

• Eight oil spraying jets, one for the underside of each piston, keeps cylinder and pistons cool and lubricated. The extra layer of oil also dampens any noises emanating from the pistons.

The LT4 is assembled in the U.S. at the new Performance Build Center at GM’s Bowling Green Assembly Plant and at GM’s Tonawanda engine plant in New York. It is matched with a standard seven-speed manual transmission or an all-new, paddle-shift eight-speed automatic transmission also built in America (Toledo, Ohio).

A convertible version of the 2015 Corvette Z06 costs $84K while a coupe goes for $79K. The standard convertible 2015 Corvette runs $55K and the standard convertible carries a $60K price tag.

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