Big Guns

TDC

Big guns

Kevin Cameron

FOUR-STROKES, TWO-STROKES-WHY NO one-stroke engines? A four-stroke needs four end-to-end piston strokes to complete its cycle, a two-stroke needs two. What would a one-stroke engine be like? Well, one-stroke engines are already completely familiar to us under their proper name: guns.

In a gun, the fuel and “air” (oxidizer) are combined chemically as the unstable gunpowder or propellant. These substances are unstable because the fuel and oxidizer arc part of the same molecule, kept from rushing violently into combination with each other by inert atoms between. Too much heat or a sharp blow may set them off. When the gun is fired, ignition is supplied not by a sparkplug but by a primer, which shoots a jet of flame into the powder, upsetting its limited stability.

In our familiar piston engines, the fuel supplied by carburetor or injector is stable, with the oxygen of the air at room temperature acting as a “spacer” to make fuel-to-oxygen collisions that much less likely. The higher the temperature rises, the faster the rate of spontaneous reaction becomes.

In a gun, once the powder is ignited, it burns quickly to create hot, highpressure gas that drives the “piston” (the bullet or projectile) up the “cylinder” (barrel). Here, the goal is to use the pressure energy of the burned propellant to accelerate the projectile to a high velocity. In a piston engine, there is a mechanism-the connecting-rod and crankshaft-by which the pressure energy is made to drive rotating machinery instead. The gun’s projectile is made quite heavy, and so acts as a kind of “linear flywheel,” storing the energy that will be used to overcome air resistance on the way to the target, and in penetrating that target. In the case of a piston engine, the goal is to transmit pressure to the crank, so all the parts in the mechanism are made as light as possible.

Guns and piston engines are made of metals whose strength is limited, so there is a close analogy between gun combustion and piston-engine combustion. In each case, the combustion pressure rise must be slow and smooth enough to avoid serious damage to the cylinder and piston. In a gun, this controlled combustion is achieved by using chemical compounds that burn at a moderate rate, by blending-in inert ma terials, and by forming the propellant into grains whose limited surface area further controls burn rate. Explosives exist whose detonation rate is several miles per second-for example HMX (can any of us say cyclotetramethyl enetetranitrarnine?), which detonates at 5.7 miles per second. Combustion pres sure in guns must be tailored to rise no higher than 25,000-50,000 psi as it ac celerates the projectile up the bore. A fast-burning explosive like HMX would burst the gun before the projec tile had time to move at all! Peak pres sure in a piston engine is only 2 to 4 percent of this, say 1000 psi.

In a piston engine, combustion is controlled to 50-250 feet per secondonly 1/200 of the speed of detonating explosives. As in the case of guns. re action rate is controlled by fuel chem istrv, and by the presence of inert nitrogen in air. Fuels such as hydro gen, acetylene or boron hydrides react with air by burning at supersonic speed. Such a fast pressure rise (at rates 10 to 100 times the normal rate) hammers out con-rod and crank bear ings, peens over piston-ring lands and erodes piston crowns. Unt~rtunately, even normal fuel-air mixtures can det onate in this way. As the unburned fuel in the cylinder is heated by corn pression and then by the combustion in the normal turbulent flame front, so-called "pre-flame reactions" occur. The rapid molecular motion of high temperature beats the weaker fuel

tiioie~uies d~dI 1. cleatIng sensitive chemical species that can. if their population rises high enough, go off by themselves. Once this happens, the presence of the sensitive fragments in the remaining unburned mixture al lows it to burn at a greatly accelerated rate-it detonates.

We prevent detonation by using fuels whose molecules are not so easily bro ken apart by heat, and with additives like tetraethyl lead (no longer legal) or \1 \`IT, that immobilize the sensitive fragments or convert them to harmless forms. I-i igh-octane gasoline is simply gasoline that better resists thermal break-up into sensitive fragments.

Similar sensitivity is displayed by explosives. improperly formulated or too long stored. \lolecules break down, creating extremely shockor friction-sensitive substances. kven normal handling can ignite them.

In an artillery piece, the `power stroke" occupies about .01 5 second; in a rifle, it takes `ho as long because the barrel is i/10 as long. Compare that with the .0025 second of a Superbike piston stroke at 1 2,000 rpm, traveling only 1.8 inches. The 155mm projectile and rifle bullet accelerate to about 2500 feet per second, the piston to unIv 80 feet per second. The shell is accelerated at about 5000 (is. the rifle bullet at 1 0 times that, but the piston accelerates at more like 3000 (is.

There is a price to pay for the ex treme simplicity of a one-stroke enine. The lifetime of a 155mm gun is unly a f~w hundred shots, or five or six seconds. The high temperature of burning propellant quickly erodes the barrel liner (big guns have replaceable liners just as many piston engines do). A rifle might last 50,000 shots, for a total lifetime of just over one minute. A 50,000-rn i Ic time-before-overhaul from a street motorcycle translates to something like 1 500-2000 hours. In the few hundred "strokes of the big gun's life, projectiles travel a mile or Iwo in the barrel, hut in the bike en Lzine, each piston travels in its cylinder ihout I/c as far as the machine itsel f~ for a total of about 1 0,000 miles. It is because the piston engine oper ites at only a moderate level of vio lence, with amenities like lubrication md cooling, that it can entertain us for such lone neriods of time.