Automotive Engine Performance

Compression-Ignition Engines

5-09 Summarize the operation of CI engines.

A CI engine is very similar to an SI engine, with the exception of the fuel and a spark plug. The fuel that is used in most CI engines is a lower-grade petroleum product that is less likely to spontaneously combust than gasoline. With a fuel that is more tolerant to high temperatures and pressures, the engine can have a higher compression ratio that increases the power output from the chemical input. This process is still a four-stroke Otto cycle, but without the aid of an ignition system, and lacking this system minimizes the number of pieces that could fail (FIGURE 5-26).

FIGURE 5-26 CI engines can come in all sizes and are designed to work for the needs of the application they will be used in.

All of this increased power does come at a cost of increased emissions from higher temps in the combustion chamber, increased noise from the combustion process, and also soot from unburnt particulate matter. Emissions and noise have been the weaknesses of CI engines: the noise has been a customer concern and the emissions have been a governmental one. The efficiencies that CI are known for are the reasons that it is heavily used in the heavy trucking/equipment industries. Moving big equipment reliably and cost effectively is the goal of any company which means CI engines are the answer. The passenger vehicle market has recently seen a new resurgence of light-duty diesel engines whose noise, emissions, and stigmas have changed, so they are now more appealing to the customer.

The newer CI engines behave like their SI counterparts in noise, emissions, and availability. What they excel in is the increased power produced from their design. This increased power comes from a heterogeneous burn, which creates power at lower speeds than an SI engine. Unfortunately, as technology advances and vehicles are engineered more to behave like the consumer wants, the higher the cost becomes, which is why most of these types of applications are more expensive than their SI competition. As energy changes within this type of engine from a chemical fuel to a mechanical motion, the efficiency of the conversion event increases to higher than that in an SI engine, which is where the benefits come from.

Thermal Efficiency of a Compression-Ignition Engine

The thermal efficiency of a CI engine is where the engine creates its higher efficiency. A CI engine is 30–35% efficient in the way it uses the potential heat input to create an output. This lean burn type of combustion process, coupled with a higher compression ratio than an SI engine is the main reason why it is the chosen type of engine for industry. The main driver of thermal efficiency in any engine is the ability to use a higher compression ratio to increase the complete burn and output power based on input. The higher compression ratio and complete organic burn of the fuel, the more efficient the engine is considered.

Gasoline Direct-Injection Engine

The next evolution of the gasoline engine is GDI. This is a combination of both an SI engine and a CI engine. Using higher compression ratios and more precise control over fuel injection, the ability to produce more power with less emissions is possible. This type of engine puts the injector into the cylinder and allows for multiple injection events to control when combustion is happening. Because the engine is just compressing air until the last minute. The design of the engine is somewhat different than a conventional engine, in that the pistons and the cylinder head are more of a requirement to help with complete combustion within the cylinder (FIGURE 5-27). The design of the piston head creates turbulence, which causes the fuel to atomize as it hits the machined piston head, resulting in a more efficient combustion process. This process allows for a more complete combustion, which lowers emissions, improves performance, and increases the longevity of the engine.

FIGURE 5-27 The GDI engine has some minor differences from a typical SI engine. The major differences are the design of the piston face and the position of the fuel injector.

Homogeneous Charge Compression-Ignition Engine

The homogeneous charge compression-ignition engine (HCCI) is a new technology that uses the CI engine’s efficient combustion with the ease of operation of an SI engine. Instead of using a lower-grade fuel oil such as diesel fuel, HCCI engines use gasoline, which is known to have better pollution and operational features (FIGURE 5-28). The use of gasoline instead of a diesel fuel decreases the temperature that is required to have a combustion event which lowers the NOx present after the combustion process. Operating under the Otto four-stroke process, fuel is introduced into the cylinder from an injector that is mounted in the cylinder head. As the piston completes its intake stroke, sucking in air and combining that air with fuel, the compression stroke starts at BDC. The piston is traveling back up the cylinder, building pressure as it does, to the point where the piston reaches BTDC, the pressures that have built up cause the fuel to spontaneously combust, starting the combustion process. This process produces a lean condition that maximizes the air-fuel ratio, which promotes complete combustion with far less emissions.

FIGURE 5-28 Increasing the fuel efficiency and combustion process of the ICE occurs through homogeneous combustion.

The biggest issue is controlling the timing event for the combustion process. With conventional SI engines simply controlling injection and ignition timing could be done by the PCM. In the HCCI engine, the combustion event is not directly controlled by the PCM; it is more of a molecularly controlled event. With the combustion process happening with no spark, complete combustion happens, which limits the thermal loss from the event, thus helping to more quickly prime the cylinder for the next event. The temperature up within the cylinder can be kept up by varying valve events and introducing EGR into the cylinder.

Mazda SPCCI Engine

Mazda’s SPCCI engine version uses a spark plug to help with ignition when the conditions are not optimal for HCCI combustion (FIGURE 5-29). The types of situations would be cold start, when ambient temperature is not high enough to promote combustion, and high-load situations, where the HCCI is not as reliable. When the spark plug does operate, it will work to change the compression ratio to a lower one that would work better with SI. To allow for the change from HCCI to SI, the engine will change its timing through the PCM and valve timing to allow it to operate simultaneously on both types of systems. Mazda is using the spark plug to start the combustion event—to increase combustion chamber pressures to the point that the fuel around the spark event will spontaneously combust, thus starting the HCCI event. This change from one to the other is where concerns could arise, so technicians must understand this process when they diagnose the potential concerns with this type of system.