Gasoline Engines Versus Diesels

Comparing spark ignition with compression ignition.

Kawasaki KLR650 prototype
Make mine a diesel? In 2008, Cycle World tested a Hayes Diversified Technologies prototype based on a Kawasaki KLR650. “You’d never describe the engine as ‘snappy,’ ” the author noted, “but it does grind out the power with only a little less sizzle than the stock gas KLR.”Jeff Allen

We gotta know this stuff in order to be proper motorheads. There is a whole spectrum of possible combustion systems out there, of which the familiar gasoline-fueled spark-ignition four-stroke is just one example.

In a spark-ignition engine, fuel and air are mixed before ignition. In the port fuel injection employed on most modern motorcycle engines, this means injecting fuel as a spray of droplets into the air stream as it passes through the throttle bodies. That combination then passes into the cylinder head, through the intake valve(s), and into the cylinder during its intake stroke (piston descending). Port injection thus gives quite a long time—much of the intake stroke and nearly all of the compression stroke—for fuel droplets to evaporate and form an ignitable mixture of gasoline vapor and air.

Shortly before the piston reaches Top Dead Center (TDC) on its compression stroke, the ignition system sends a hot spark across the electrodes of the spark plug. If the air-fuel mixture is in the range of 18:1 to 10:1, a flame kernel results with high probability. The turbulent motion of the charge rapidly shreds and distributes this flame kernel to produce a flame front. The flame front advances from spark plug to cylinder wall, converting the air-fuel mixture into very hot, high-pressure combustion gas.

The ignition spark is timed to produce peak combustion pressure just as the piston’s very slow motion near TDC accelerates downward on the power stroke at around 11 degrees After Top Dead Center (ATDC). As this hot, high-pressure gas expands against the descending piston, its pressure performs work, which is force times distance. This combustion-gas expansion causes its pressure and temperature to fall rapidly. By the time the exhaust valve(s) begin to lift some 50 degrees before Bottom Dead Center (BDC), gas pressure has fallen low enough that it can no longer perform useful work and so it is released from the cylinder into the exhaust pipe. We hear the sudden expansion of that residual pressure as exhaust sound.

Now the piston again rises—this is the exhaust stroke—pushing the exhaust gas remaining in the cylinder into the exhaust pipe. The cycle of intake, compression, power, and exhaust repeats.

The Limits Of Power: Detonation

A major potential problem of spark ignition is that it generates a flame front. That flame front, hot as it is, expands against the remaining unburned charge, compressing and heating it. What we’d like is for the flame front to advance rapidly and to consume all of the unburned charge. But if, for various reasons, the heating of the unburned charge is prolonged—slow combustion, too-early ignition, etc.—the very last parts of it to burn may undergo heat-driven chemical change that converts it into a sensitive explosive. Tiny volumes of such overheated charge, igniting spontaneously at the very end of combustion, may burn at sonic speed, generating shock waves that we hear as combustion knock or detonation. It is detonation that sets the upper limit of safely usable compression ratio at around 12:1 or 13:1 in liquid-cooled engines with moderate-sized cylinders, and at 10:1 or 11:1 in air-cooled engines with large cylinders.

In the days of carburetors and magnetos, motorcycle race tuners seeking maximum performance had to push tuning variables—fuel mixture, compression ratio, timing—very close to detonation, so they were all too familiar with it and its destructive effects. The coming of digital engine controls has made all that unnecessary.

Modern Motorcycle Engines Are Protected Against Detonation

Detonation is extremely rare in modern computer-controlled engines because stored digital maps accurately control air-fuel mixture and spark timing, making detonation all but impossible. But if fuel of poor antiknock rating (low octane number) gets into the system, the last line of defense is the combination of a knock sensor and an ignition-retard system that will positively suppress any detonation that occurs.

The Diesel Engine

The diesel engine works in a quite different way. A first difference is that diesels operate un-throttled; their intakes are fully open at all times. The power of diesel engines is controlled solely by the amount of fuel injected in each cycle. The very high compression ratio of a typical open-combustion-chamber diesel is in the range of 16:1 to 17:1. Old prechamber engines employed compression ratios as high as 21:1 to enable cold-starting.

As a diesel’s piston nears TDC on its compression stroke, the temperature resulting from such high compression is enough that any liquid fuel injected into it will quickly ignite. This is compression ignition; for cold-starting, the assistance of a glow plug may be required.

Fuel is sprayed directly into the combustion chamber at extremely high pressure (25,000 to 30,000 psi), forming tiny droplets moving at high speeds around 1,000 feet per second. This high speed is necessary to make the droplets penetrate the dense compressed air in the cylinder. Although the diesel ignition-and-combustion process has many interesting details, it’s enough to say here that the hot compressed air in the cylinder ignites the fuel shortly after its injection. To avoid the formation of smoke, only enough fuel to consume about 80 percent of the oxygen in the air charge is injected.

Diesels typically use about 30 percent less fuel than do spark-ignition engines of equal horsepower.

Because of their very high compression ratio, diesels produce high torque and operate at lower rpm, which is limited by the time it takes to inject and burn the necessary fuel. To enable high power to be generated at such low rpm, four-stroke diesels are usually turbocharged.

Diesels typically use about 30 percent less fuel than do spark-ignition engines of equal horsepower. There are several reasons for this:

  1. The higher an engine’s compression ratio, the higher the peak combustion pressure that results, the greater the fraction of the fuel’s energy that does work on the pistons, and the less that is rejected to coolant or goes out the exhaust pipe (heaters in diesel-powered vehicles are notoriously weak).
  2. By operating unthrottled, a diesel engine avoids most of the pumping loss that makes the part-throttle operation of spark-ignition engines less efficient.
  3. By making its power more through pressure and less through rpm, a diesel engine’s mechanical friction is reduced.

Diesel combustion does not form a flame front; ignition occurs wherever fuel droplets and hot air come together. Because there is no flame front, there can be no prolonged heating of premixed volumes of fuel-air mixture, so there can be no detonation. Freed of the limiting effect of detonation, a diesel’s compression ratio can be set much higher than that of a spark-ignition engine.

How Fuel Droplets Lead To Combustion

Because the fuel in a droplet is not mixed with air, it cannot burn. Instead, what happens is a diffusion process: Fuel vapor evaporates from the droplet’s surface and diffuses radially away from the droplet. Meanwhile, oxygen from the surrounding air diffuses inward, toward the droplet. Where the two mix and are hot enough to ignite, a so-called “diffusion flame” occurs. It continues until either the droplet is consumed or the cooling of combustion gas by expansion (as the piston descends) extinguishes the flame. The charred remains of the fuel droplet may now participate in soot formation.

Gas refill
Fill ’er up: HDT claimed a dry weight of less than 400 pounds and a range of more than 600 miles for its single-cylinder diesel . The Southern California-based company also produced a military-spec model—color? Olive Drab, of course—known as the HDT M1030M1.Jeff Allen

What Is Gasoline Direct Injection?

Confusion can occur when we hear about spark-ignition engines that have direct cylinder fuel injection (GDI) instead of the more common port injection. GDI engines are not diesels; they just use a different means of creating a premixed air-fuel mixture of normal proportions that is then ignited in the normal way by a spark. Since they rely on a premixed charge, intake airflow must be correctly proportioned to the amount of fuel injected by use of a throttle. The advantage of GDI is that significantly more air can be drawn into engine cylinders if that air is not carrying fuel vapor. German spark-ignition gasoline-fueled aircraft engines of World War II employed GDI. A special problem of GDI is that less time is available for fuel evaporation, imposing a degree of rpm limitation on engines using it.

What Is HCCI?

Another confusion is normal when we hear about Homogeneous Charge Compression Ignition (HCCI) engines. In complete contrast to diesels, which draw in and compress pure air enough to sparklessly ignite fuel injected into it, an HCCI engine operates on a homogeneous (premixed) air-fuel charge that is throttled. In one form of HCCI, enough hot exhaust gas from the previous cycle is retained in the cylinder to cause the fresh charge to spontaneously ignite in a great many places at once as the piston nears TDC on its compression stroke. This is tricky because fuel, air, and exhaust gas must be so proportioned that ignition occurs at the desired point in the cycle, despite changes in load and rpm. Also, at idle and low throttle, HCCI typically produces too little heat to ignite by compression alone, so a spark may be necessary to help the process along. Spark ignition may also be necessary at or near full throttle, when the volume of cool fresh charge is too great to be heated to ignition by retained exhaust gas. HCCI has the potential to deliver high efficiency but is a delicate balancing act.

Why Not More Diesel Motorcycle Engines?

While we have seen some low-powered novelty bikes powered by diesel engines taken from other applications, the diesel cycle has not made much headway in the two-wheel market. Why not?

When I put this question to a diesel engineer, his response was to point out that it’s very difficult to combine the diesel’s very vigorous torque pulses with the one or two cylinders of many motorcycles. Smoothing out such pulses, because it would require so massive a flywheel, might not be practical on a light vehicle.