The Applied Physics of Gasoline Engines, Part 2

Also see: To enhance our automotive appreciation, we continue here our two-part dissection of the four-stroke gasoline engine, using the one-cylinder mower motor as our case study of the physics principles that make engine operation possible. In Part 1 of this series [3] we examined the engine’s gross anatomy. We discussed fuel, lubrication, and cooling systems, as well as the upper limit on an engine’s thermodynamic efficiency. Recall how the explosion of the air–fuel mixture drives the piston down the cylinder, and how the connecting rod and crankshaft converts the piston’s linear motion into rotary motion. A camshaft coupled to the crankshaft by timing gears opens valves to allow the air–fuel mixture to enter and exhaust gases to exit the combustion chamber. One cycle of operation consists of four strokes: intake, compression, power, and exhaust. At the transition between the compression and power strokes the spark plug fires, exploding the air–fuel mixture to drive the piston down the cylinder. In this installment of “Elegant Connections” we examine how the spark is generated and delivered at the crucial moment. The necessary developments are a tribute to the minds behind the Faraday-Lenz law of magnetic induction.

The business end of the spark plug (Fig. 1) extends into the combustion chamber and features a gap between electrodes. A spark jumps across the gap—a bolt of lightning in miniature—when the plug receives a sudden jolt of voltage sufficient to ionize air. The breakdown potential gradient of air is about 3 MV/m. The shop manual calls for the motor mower’s spark plug gap to be set at 0.030 inches.[4] The delivered voltage pulse peaks at about 10 kV and creates a momentary electric field between the electrodes on the order of 10 MV/m, sufficient to produce a fat blue spark. The task of producing this voltage falls to the magneto.

Bolted to the block, the magneto consists of a primary coil of copper wire wrapped around the base of a U-shaped laminated iron armature. The ends of the U are located about 0.012 in. from the perimeter of the aluminum alloy flywheel, which carries a strong embedded permanent magnet and a nonmagnetized counterweight opposite the magnet for balance (Fig. 2). As the magnet sweeps by the laminated armature, the primary coil picks up a changing magnetic flux, which induces a voltage or electromagnetic force that drives a current in the primary coil. The flywheel magnet produces in the primary coil a peak emf woefully inadequate to create the spark. Therefore the primary coil is surrounded by a secondary coil to make a transformer. Let us trace the essential events throughout the operation of this magneto.

The current in the primary coil makes its own magnetic field with a flux that fills the secondary coil. To maximize the rate of change of magnetic flux in both coils, the current in the primary coil is abruptly cut off when it reaches its maximum value. This is accomplished by a switch composed of breaker points (or points for short) that open and close like clothespins (the kind of clothespin with a spring; the “points” are the small faces of the “clothespin” that touch when closed). The points are wired in series between the primary coil and ground (Figs. 3, 4). The abrupt change of the primary coil’s current caused by the opening of the points induces within the primary coil a new emf of about 170 V, which gets amplified into the output voltage of the secondary coil. Recall that in a transformer the ratio of the voltage coming out of the secondary coil to that of the input into the primary coil is ε2/ε1 = A2N2/A1N1, where N denotes the number of turns in a coil, A the area enclosed by a turn, and ε the emf. For the mower motor the large number of turns in the second coil yields ε2/ε1 ≈ 60, inducing a secondary voltage output of around 10 kV. This strong voltage is carried to the spark plug by a heavily insulated spark plug wire. The lower end of the spark plug, screwed into the head, forms the ground and thereby completes the secondary circuit. The high-voltage pulse creates the spark, which jumps the gap between the plug’s electrodes to ignite the air–fuel mixture.

Left to itself the 170 V emf on the primary coil could arc across the points while they are open (their gap specification is 0.020 in.), which would smooth out the interruption of the primary coil’s current and negate the reason for the points in the first place. The remedy is to place a capacitor, or condenser, in parallel with the points, giving the primary coil’s emf a capacitor to charge while the points are open (Fig. 4). When the points are open the primary coil and condenser form an LRC circuit (consisting of a resistor, an inductor, and a capacitor). When the points close, the condenser merely discharges to ground.

The points are opened and closed by a cam-and-spring arrangement. On our mower motor the points live adjacent to the flywheel end of the crankshaft, between the crankcase cover plate and the flywheel. One half of the points assembly (like half a clothespin) is held fixed while the other half rides on a lever arm that is held by a spring against the crankshaft’s end and contacts the crankshaft through a rubbing block. Machined into the crankshaft is a flat spot that, when passing under the rubbing block, allows the points to spring apart, routing the path for a closed circuit exclusively through the condenser. The instant in the four-stroke cycle when the spark plug will fire is determined by the location of the flywheel magnet relative to the crankshaft, which is held fixed. This is accomplished with a rectangular key that fits into aligned slots machined into both the crankshaft and the flywheel.

As the engine speed increases, for optimal performance the timing of the spark must be advanced so that the ignition starts sooner in the cycle and the combustion of the compressed air–fuel mixture will be complete. But because it is designed to be as simple as possible, the ignition timing on our mower motor engine is held fixed at a compromise setting.

Multicylinder engines require a distributor that sends sparks sequentially to several spark plugs. Mechanically, the distributor houses a rotating distributor shaft driven by a gear on the camshaft and topped with a rotor that is perpendicular to the shaft and extends out from the shaft’s center like the hand of a clock. As the shaft spins, the rotor also spins. On multicylinder engines the points and condenser reside inside the distributor housing, which is topped by the distributor cap. The points are opened by a cam on the distributor shaft. On an engine with N cylinders (assuming one spark plug per cylinder) the Medusa-like distributor cap has N+1 wires sticking out of its top. One wire carries current from the magneto (or coil) to the cap’s center to the base of the rotor. The other N wires plug into connections spaced evenly around the cap, each wire connecting a cap junction to a spark plug. Electric current from the magneto (or coil) enters the distributor cap at a center plug-in and passes to the rotor, which turns like a clock hand beneath the cap to direct the current to the next spark plug scheduled to fire.

As mentioned above, for optimal performance the timing of the spark should be advanced as the engine rpm increases. On engines with distributors this can be achieved by rotating the distributor body about the axis of the distributor shaft. Then the points open a bit sooner, and the spark is sent a bit earlier in the transition from the compression stroke to the power stroke. The distributor is rotated manually on early cars (models built from around 1900 and into the 1920s). To operate one of these vehicles the driver moves a lever mounted on the steering wheel hub and linked to the distributor. Cars from the 1930s into the 1970s feature vacuum advance, with the carburetor venturi connected by a thin tube, the vacuum line, to a diaphragm on the side of the distributor. When the engine rpm increases, the drop of pressure in the venturi pulls on the distributor diaphragm and rotates it. By the late 1970s the spark timing in cars was controlled electronically. The point-condenser system was replaced by solid-state electronics. Sensors detect the crankshaft position and engine rpm, and a computer called the control module (typically found beneath the distributor cap) performs a calculation and advances or retards the spark accordingly.[5]

We have been discussing events that occur within an engine already running. What has to happen for the engine to start running? To achieve a sufficiently potent spark for the mower motor to run self-sustainingly, the crankshaft needs to spin at least 90 rpm. Early cars, trucks, tractors, and airplanes had magneto ignition and were started by hand-cranking the motor. Motorcycles through the 1960s featured a ratcheted kick starter, and some new ones still do.

Beginning with the 1912 Cadillac, the flywheel was given an additional job as part of a “self-starting” system.[6] This system consists of a starter, which is an electric motor that includes a pinion gear on the back of the armature and a solenoid above the pinion gear. A set of outward-pointing teeth, the ring gear, is mounted on the flywheel’s perimeter. Turning the ignition key to “start” closes the ignition circuit that serves the coil and spark plugs, and also sends electric current from the battery to the starter and its solenoid. The magnetic field in the solenoid levers the pinion gear back to engage the ring gear so the starter can rotate the crankshaft. After the engine fires up, the driver releases the “start” setting on the ignition switch, leaving the ignition circuit activated but opening the starter circuit. A spring snaps the pinion gear back to its original location free of the ring gear. But our mower motor does not need such frills. A cord, wrapped around a spool with a ratchet on a spring to rewind it (Fig. 2), is all you need to start a one-cylinder lawn mower motor!

To start a car, you may also need to adjust a valve called the choke, which controls the amount of air mixed into the fuel. How is gasoline mixed with air? The central player here is the carburetor. Air enters the engine through the air cleaner, which filters out dust. The cleansed air passes into the throat of the carburetor, which narrows down to a restriction, the venturi. Here the air speeds up (recall the equation of continuity). A throttle valve in the venturi controls the restriction’s effective diameter and thus the speed at which the engine runs. According to Bernoulli’s equation, faster air has lower pressure, and thus gasoline in the carburetor’s reservoir, or float bowl, held at ambient atmospheric pressure gets pushed through a small orifice into the venture, where it vaporizes. From there the air–fuel mixture travels down the intake manifold to the passage in the side of the block [or in the head on overhead valve/overhead camshaft (OHV/OHC) engines], where it enters the combustion chamber when the intake valve opens.

The ratio of air to fuel is crucial to engine performance. Too much fuel (“running rich”) and the spark plugs foul as the incomplete combustion allows carbon residue to accumulate on the electrodes. An excessively rich mixture reveals itself by black smoke coming out of the exhaust. Too little fuel (“too lean”) and not only will power drop by fuel starvation, but the valves may eventually burn because gasoline helps lubricate the valves. For cold start-ups the engine needs a richer air–fuel mixture. Partially closing the choke reduces the volume of air entering the carburetor, thereby enriching the mixture. After the engine warms up the choke is fully opened.

When the needle valve in the carburetor’s float bowl opens, fuel enters from the gas tank thanks to gravity. (For most cars built before the 1930s, e.g., in the Ford Model T, the fuel tank sits above the dashboard.) Cars built since the 1920s use a fuel pump to push petrol from the fuel tank to the engine. Mechanical fuel pumps operate off the camshaft; some cars use an electric fuel pump near or inside the fuel tank. Most late-model cars and some motorcycles now use fuel injection instead of carburetors, which squirts a jet of fuel directly into each combustion chamber immediately before the spark plug fires.

If the anatomy of the gasoline engine was foreign to some readers at the beginning of Part 1 of this article, I hope the passage included below now holds meaning. This text describes the inner workings of a Ferrari, but I could have selected any car as an example; they’re all marvels of engineering. I chose this passage only because the author was particularly eloquent in his descriptions.

A Ferrari is, above all, an engine. Without its superbly responsive, seemingly limitless-revving power plant, a Ferrari would be just another of a number of beautiful, well built, good handling European sports or Gran Turismo vehicles. With its engine, it becomes an animal thing, a fascinating blend of machinery and emotion that is certainly responsible for maintaining the Ferrari’s mystique in spite of all its temperament. Ferrari engines have been built in many configurations. . . . But the Ferrari engine, in the heart and mind of the enthusiast, is the V-12. And all of the many variants of this engine stem from one, the 1497-cc, single overhead camshaft V-12 designed in 1946 for Ferrari by Ing. Gioacchino Colombo.

. . . Its twelve cylinders were set in a 60° V with a 20-mm offset between banks so that side-by-side connecting rods could be used. The distance between bore centers was 90 mm. Cylinder heads, block, and crankcase were cast of aluminum alloy. Cast-iron wet cylinder liners were shrunk in and further held in compression against the block by the cylinder heads . . . At the top, compression was sealed by soft, individual copper-asbestos rings for each cylinder.

The cylinder heads held single overhead camshafts which ran in six bearings and actuated finger-follower rocker arms. Valves were closed by two hairpin springs per valve, in order to keep them short and thus the reciprocating mass of the valve train low. Exhaust and inlet valves were inclined at an included angle of 60°, their faces forming a nearly hemispherical combustion chamber, which was further modified to allow the insertion of a 14-mm spark plug on the inlet side. The plug placement and the notion that the engine would be supercharged in its maximum performance version led to the adoption of six intake ports (three per cylinder head) . . .

Ignition for the original engines was provided by twin horizontal magnetos driven from the rear of the camshafts. Inter Auto’s announcement of the new Ferrari engine stated that it displaced 1496.77 cc, had bore and stroke dimensions of 55 × 52.5 mm, 8.0:1 compression ratio, and three Weber 30 DCF carburetors. It developed 72 bhp at 5600 rpm. . . . Maximum speed was claimed to be 155 km/h, or 95 mph.

The attributes of this classic high-performance engine layout which has served Ferrari with distinction over two decades are worth noting here. As mentioned, the twelve cylinders provide a good balance between the development of high power vs complexity and internal friction, the latter being decreased by oversquare bore and stroke ratios. The overhead-camshaft layout and light valve gear have made it a high-revving engine and it has excellent exhaust porting. The years have seen its displacement more than doubled and its output quadrupled.[7]

It is instructive to remember that Gioacchino Colombo designed the first Ferrari engine without computers. He did it with a pencil, paper, slide rule, intuition, and experience. Today, his engine’s legacy includes the 2014 Ferrari F12 Berlinetta. Its V-12 engine produces 731 hp.[8]

Regardless of whether you drive a Ferrari or, more likely, a family sedan, a rugged work truck, or a humble econobox, you should appreciate your machine and those who made it. Treat every machine with respect.[9] An essential expression of respect is the practice of good maintenance.
In automotive maintenance, as in most areas of worthwhile endeavor, caring is the most important tool. From that everything else will follow. For instance, if you keep your car clean, your relationship with it will evolve quite differently than if you allow the interior to fill with fast-food debris and the exterior to accumulate sedimentary layers of grime. Motor cleanliness means maintaining the proper quantity of clean oil in the crankcase, cleaning the air filter, keeping the spark plug electrodes clean and properly gapped, and, for the air-cooled mower motor, hosing grass clippings and dust off the cooling fins after each mowing (but not while the engine is sizzling hot and experiencing thermal expansion). After hosing the motor, start it up and let it run for a few seconds to blow out any water that may have entered through the muffler. Do the mower a favor and store it out of the weather. These little motors ask so little but give so much. If you take care of the one under your watch it will outlive you.
While each individual machine deserves respect, we should also be aware of the collective costs and the consequences of several hundred million engines operating on our planet every day. The ecosystem also deserves respect! The Earth does not need us, but oh how we need the Earth. Like other kinds of addicts, we have grown excessively dependent on our cars. Although I love to drive, I am happy to spare the miles on my machine whenever I can take the subway. (Where did all the streetcars go?[10]) Given the choice, I prefer to save the car for weekend excursions.
My old car has been hinting that it could use a tune-up. The garage awaits. //

[1] Floyd Clymer, Those Wonderful Old Automobiles (Bonanza Books, New York, NY, 1953).
[2] See Freeman Dyson, Disturbing the Universe (Basic Books, 1979), pp. 104–105.
[3] “Elegant Connections in Physics: The Applied Physics of Gasoline Engines, Part 1: Mechanics and Thermodynamics,” The SPS Observer, Spring 2014, pp. 26–31.
[4] Specifications for the single-cylinder Briggs & Stratton come from Paul Dempsey, How to Repair Briggs & Stratton Engines (Tab Books, Blue Summit, PA, 1978).
[5] The electromechanical points-condenser system is not as efficient as the electronic control module. However, there are trade-offs. When the points fail, they usually give warning. The engine runs rough, but you still get home. In contrast, in my experience when a control module fails it does so without warning, instantly and irreversibly switching off the ignition.
[6] The monthly magazine of the Cadillac & LaSalle Club is proudly called The Self-Starter.
[7] Warren W. Fitzgerald and Richard F. Merritt, Ferrari: The Sports and Gran Turismo Cars (Bond Publishing Co., Newport Beach, CA, 1972), pp. 13–14. This passages uses “bhp” for “brake horsepower” instead of “hp” for “horsepower” (1 hp = 746 W). The two terms are almost interchangeable. The hp is a unit of power; bhp is the power delivered at the engine’s output shaft.
[8] www.motortrend.com/roadtests/exotic/1309_2014_ferrari_f12_berlinetta_exc...
[9] To this day I cannot watch a “car bash” at fundraising events. I have trouble understanding how anyone can take delight in gleefully destroying something that they cannot build themselves. Such disrespect, with effects indistinguishable from contempt, easily extends to other things besides cars.
[10] In 1949 General Motors, Firestone, Standard Oil, Phillips Petroleum, and Mack Truck corporations were found guilty of antitrust violations for buying electric streetcar lines in order to
replace them with buses. Other factors were at work too, of course. For an overview with references to primary sources, see http://en.wikipedia.org/wiki/General_Motors_streetcar_conspiracy .
[11] The engine cadaver lab was described in “The Applied Physics of Gasoline Engines, Part 1: Mechanics and Thermodynamics,” The SPS Observer, Spring 2014, pp. 26–31; “Science and Service, Appreciation and Awareness,” Radiations, Fall 2012, pp. 16–23, 31; “Motorcycle Maintenance and the Art of Physics Appreciation,” Radiations, Fall 2007, pp. 5–11; and The SPS Observer, Fall 2007, pp. 1, 3–5.