The Notorious Nitro Engine
The Notorious Nitro Engine has put fear into hearts of many professional drag racers at one time or another. But this unique fuel-burning engine is what makes drag racing what it is today. Fuel, Fire & Fury. Read on about what makes this monster roar!
Like many other motor sport formulas originating in the United States, the NHRA favors heavy restrictions on engine configuration, rather than technological development. This restricts the teams to using many decades old technologies.
The engine used to power a Top Fuel Dragster or Nitro Funny Car has its roots in the second generation Chrysler Hemi 426 "Elephant Engine" made 1964-71. Although the nitro engine is built exclusively of aftermarket parts, it retains the basic configuration with two valves per cylinder activated by pushrods from a centrally-placed camshaft. The engine has hemispherical combustion chambers, a 90-degree V angle; 4.8" bore pitch and a .54" cam lift. The configuration is identical to the overhead valve, single camshaft-in-block "Hemi" V-8 engine which became available for sale to the public in selected Chrysler Corporation (Dodge, DeSoto, and Chrysler) automotive products in 1952.
The NHRA competition rules limit the displacement to a 500 cubic inch (8194 cc). A 4.19" (106.4 mm) bore with a 4.5" (114.3 mm) stroke are customary dimensions. Larger bores have been shown to weaken the cylinder block. Compression ratio is about 6.5:1, as is common on engines with overdriven (the supercharger is driven faster than the crankshaft speed) superchargers.
The block is CNC machined from a piece of forged aluminum. It has press-fitted ductile iron liners. There are no water passages in the block which adds considerable strength and stiffness. Like the original Hemi, the racing cylinder block has a long skirt (to reduce piston "rocking" at the lower limit of piston travel); there are five main bearing caps which are fastened with aircraft-standard-rated steel studs; with additional reinforcing main studs and side bolts. There are three approved suppliers of these custom-made after-market blocks, from which the teams may choose.
The cylinder heads are CNC-machined from aluminum billets. As such, they have no water jackets and rely entirely on the incoming air/fuel mixture for their cooling. The original Chrysler design of two large valves per cylinder is used. The intake valve is made from solid titanium and the exhaust from solid Nimonic 80A or similar. Seats are of ductile iron. Beryllium-copper has been tried but its use is limited due to cost. Valve sizes are around 2.45" (62.2 mm) for the intake and 1.925" (48.9 mm) for the exhaust. In the ports there are integral tubes for the push rods. The heads are sealed to the block by copper gaskets and stainless steel o-rings. Securing the heads to the block is done with aircraft-rated steel studs.
The camshaft is billet steel, made from 8620 carbon steel or similar. It runs in five oil pressure lubricated bearing shells and is driven by gears in the front of the engine. Mechanical roller lifters ride atop the cam lobes and drive the steel push rods up into the steel rockers that actuate the valves. The rockers are of roller type on the intake side; high pressures on the exhaust limits its use to the intake side only. The steel roller rotates on a steel roller bearing and the steel rocker arms rotate on a titanium shaft within bronze bushings. Intake rockers are billet while the exhausts are investment cast. The dual valve springs are of coaxial type and made out of titanium. Valve retainers are also made of titanium, as are the rocker covers.
Billet steel crankshafts are used; they all have a cross plane a.k.a. 90-degree configuration and run in five conventional bearing shells. 180-degree crankshafts have been tried and they can offer increased power, even though the exhaust is of open type. A 180 degree crankshaft is also about 10 kg lighter than 90 degree crankshaft, but they create a lot of vibration. Such is the strength of a top fuel crankshaft that in one incident, the entire engine block was split open and blown off the car during an engine failure, and the crank, with all eight connecting rods and pistons, was left still bolted to the clutch.
Pistons are of forged aluminum, 2618 alloy. They have three rings and aluminum buttons retain the 1.156" x 3.300" steel pin. The piston is anodized and Teflon coated to prevent galling during high temperature operation. The top ring is an L-shaped Dykes ring that provides a good seal during combustion but a second ring must be used to prevent oil from entering the combustion chamber during intake strokes as the Dykes-style ring offers less than optimal combustion gas sealing. The third ring is an oil scraper ring whose function is helped by the second ring. The connecting rods are of forged aluminum and do provide some shock damping, which is why aluminum is used in place of titanium, because titanium connecting rods transmit too much of the combustion impulse to the big-end rod bearings, endangering the bearings and thus the crankshaft and block. Each con rod has two bolts, shell bearings for the big end while the pin runs directly in the rod.
The supercharger is a 14-71 type Roots blower. It has twisted lobes and is driven by a toothed belt. The supercharger is slightly offset to the rear to provide an even distribution of air. Absolute manifold pressure is usually 3.8-4.5 bar (56-66 PSI), but up to 5.0 bar (74 PSI) is possible. The manifold is fitted with a 200 psi burst plate. Air is fed to the compressor from throttle butterflies with a maximum area of 65 sq. in. At maximum pressure, it takes approximately 800-1500 horsepower to drive the supercharger.
These superchargers are in fact derivatives of General Motors scavenging-air blowers for their two-cycle diesel engines, which were adapted for automotive use in the early days of the sport. The model name of these superchargers delineates their size; i.e. the once commonly used 6-71 and 4-71 blowers were designed for General Motors diesels having six cylinders of 71 cubic inches each, and four cylinders of 71 cubic inches each, respectively. Thus, the currently used 14-71 design can be seen to be a huge increase in power delivery over the early designs.
Mandatory safety rules require a secured Kevlar-style blanket over the supercharger assembly as "blower explosions" are not uncommon. The absence of a protective blanket exposes the driver, team and spectators to shrapnel in the event that nearly any irregularity in the induction of the air/fuel mixture, the conversion of combustion into rotating crankshaft movements, or in the exhausting of spent gasses is encountered.
The oil system has a wet sump which contains 16 quarts of SAE 70 mineral or synthetic racing oil. The pan is made of titanium or aluminum. Titanium can be used to prevent oil spills in the event of a blown rod. Oil pressure is somewhere around 160–170 psi during the run, 200 psi at start up, but actual figures differ between teams.
Fuel is injected by a constant flow injection system. There is an engine driven mechanical fuel pump and about 42 fuel nozzles. The pump can flow 100 gallons per minute at 8000 rpm and 500 PSI fuel pressure. In general 10 injectors are placed in the injector hat above the supercharger, 16 in the intake manifold and two per cylinder in the cylinder head. Usually a race is started with a leaner mixture, then as the clutch begins to tighten as the engine speed builds; the air/fuel mixture is enriched. As engine speed builds pump pressure the mixture is made leaner to maintain a predetermined ratio that is based on many factors, one of which is primary one of racetrack surface friction. The stoichiometry of both methanol and nitromethane is considerably greater than that of racing gasoline, as they have oxygen atoms attached to their carbon chains and gasoline does not. This means that a nitro engine will provide power over a very broad range from very lean to very rich mixtures. Thus, to attain maximum performance, before each race, by varying the level of fuel supplied to the engine, the mechanical crew may select power outputs barely below the limits of tire traction. Power outputs which create tire slippage will "smoke the tires" and the race is often lost.
The air/fuel mixture is ignited by two 14-mm spark plugs per cylinder. These plugs are fired by two 44-ampere magnetos. Normal ignition timing is 58-65 degrees BTDC. (This is dramatically greater spark advance than in a gasoline engine as "nitro" and alcohol burn far slower.) Directly after launch the timing is typically decreased by about 25 degrees for a short time as this gives the tires time to reach their correct shape. The ignition system limits the engine speed to 8400 rpm. The ignition system provides initial 50,000 volts and 1.2 amperes. The long duration spark (up to 26 degrees) provides energy of 950 millijoules. The plugs are placed in such a way that they are cooled by the incoming charge. The ignition system is not allowed to respond to real time information (no computer-based spark lead adjustments), so instead a timer-based retard system is used.
The engine is fitted with open exhaust pipes, 2.75" in diameter and 18" long. These are made of steel and fitted with thermocouples for measuring of the exhaust temperature. They are called "zoomies" and exhaust gases are directed upward and backwards. Exhaust temperature is about 500 °F (260 °C) at idle and 1796 °F (980 °C) by the end of a run. A night run provides visual excitement with slow-burning nitromethane flames many feet above this screaming spectacle of acceleration. A "good run" is over in just 4.5 seconds, the noise ends, and braking parachutes are seen in the distance, after a speed of over 325 miles per hour (523 km/h) has been reached.
The engine is warmed up for about 80 seconds. After the warm up the valve covers are taken off, oil is changed and the car is refueled. The run including tire warming is about 100 seconds which results in a "lap" of about three minutes. After each lap, the entire engine is disassembled and examined, and worn or damaged components are replaced.
Measuring the power output of a nitro engine directly is not feasible. This is not, as is sometimes stated, because no dynamometer exists that can measure the output of a nitro engine; in reality, dynamometers capable of measuring tens of thousands of horsepower at the appropriate shaft speeds are in widespread use. Rather, it is because a nitro engine cannot be run at its maximum power output for more than about 10 seconds at a time without overheating (or perhaps exploding) as would be necessary to take a reliable power reading. Instead, the power output of the engine is usually calculated based upon the car's weight and its performance. The calculated Power output of these engines is most likely somewhere between 7000 and 8500 horsepower, with a torque output of 6000 lb-ft.