Combustion In A Diesel Engine A more accurate technical term for the Diesel engine, which bears the name of its inventor, is a “Compression Ignition Engine”. This term is given to it since combustion in a diesel is initiated through the heat developed by compression pressures. It can be observed that when a gas is compressed its temperature increases. The temperature and pressure can be predicted mathematically using Boyle’s Law. The high compression ratio of a diesel engine facilitates high pre-ignition temperatures necessary to combust diesel fuel. (200- 250 degrees C are the minimum temperatures required to burn diesel fuel).

Typically a diesel engine will develop temperatures in the range of 800 – 1200F at the end of it’s compression stroke. These pre-ignition temperatures will vary as cylinder pressure changes with engine speed and turbo boost. Intake air temperature will also change the pre-ignition temperatures. The efficiency of a diesel engine is due to its high compression ratio. Since the engine uses high compression ratio the expansion coefficient – (which is the comparison of the combustion chamber size to the volume of combusted air and

fuel) is much higher in a diesel. For this reason the diesel will develop higher cylinder pressures yeilding greater torque and use less fuel doing so. Advantages of High Compression Ratio.

Engines with high C.R. can convert more heat energy into kinetic energy because of a greater expansion ratio.

High CR mixes air and fuel better because air molecules are more active when under pressure and heat.

Smaller combustion space is more thermally efficient retaining more heat on compression and power stroke.Better cylinder scavenging since more exhaust gases are pushed out of cylinder on the exhaust stroke due to smaller clearance volume.

Disadvantages of High C.R. CR increases above 16:1 have minimal power

increase since expansion related to piston movement is negligible.

Higher CR’s can reduce mechanical efficiency because of friction losses and strengthening of parts such as pistons, cranks, rods, etc.… making them heavier.

Head gaskets must seal better, heavier starting systems are required

Compression Ignition Combustion Chambers Two Major Compression Ignition Combustion Chamber designs are: ·Direct Injection (DI ) ·Indirect Injection (IDI ) Combustion chamber design is most significant factor determining characteristics of a diesel engine – its fuel sytem, operational characteristics and construction of major engine components & systems. A good combustion chamber design will:

• Promote efficient combustion • Limit emissions mandated for reduction (HC, NOX, CO & Particulates) • Limit noise • Provide good fuel economy • Smooth engine operation

Above: 74% Smoke Opacity – Evidence of Diesel Combustion Particulates

Direct Injection (DI) Combustion Chambers DI chambers define a category of chambers where the fuel is injected directly into a combustion chamber formed between the top of the piston and cylinder head. The injector/nozzle, intake & exhaust valves have direct access to the combustion chamber.

Compression ratios in a DI chamber are typically between 15:1 to 18:1 for most on-highway diesel engines. This produces the necessary ignition air temperatures of between 800 - 1200 degrees F.

An average pre-ignition temperature is 1000F .

Higher compression ratios produce limited increases in power and actually power diminishes after 23:1.

When using air to air after cooling and turbo charging produce a typical compression pressure is 425 - 650 psi pressure. The pressure will be significantly lower when cranking.

DI chambers use multi-orifice nozzles to atomize and distribute fuel in the combustion chamber.

High injection pressures are required to penetrate compressed air in combustion chamber, distribute and mix fuel evenly in the combustion chamber in the short period of time near the end of compression stroke.

Multi-orifice injection nozzles with nozzle have nozzle-opening pressures (N.O.P.) of 3000 - 4500psi. (Spray in pressures higher) Below: Injection and beginning of combustion in a DI chamber. Note location of valves and injector and ignition points.

Higher thermal efficiencies are achieved using DI chambers since there is little combustion surface wall area compared to combustion volume. This results in 10 - 15% better fuel efficiency. Current thermal efficiencies as high as 45% in on-highway and over 50% in slow moving industrial iengines

Two common types of DI chambers are the offset and central DI. These refer to the location of the injector in the combustion chamber. Central DI’s use four valves per cylinder and the injector located in the centre.

Offset DI’s use two valves per cylinder. Since there isn’t room to accommodate the injector in the center because of the larger valves, the injector is “leaned” to the side of the cylinder head making room for the valves.

Central DI distributes fuel more symetrically for bettter emission charachteristics. 4-valves help the engine to “breath” better and better centralized placement of the injector.

Offset injection, while less expensive to manufacture, creates uneven piston crown temperatures and thrust loads in addition to higher emissions.

DI chambers require no starting aids – (ie

glow-plugs intake heaters etc.. ). These may be present on some engines but they are usually there to minimize white smoke from miss-fires during start-up and warm-up period.

Higher cylinder pressures and longer duration

of the effectiveness of the power stroke are characteristic of DI chambers. Cylinder pressures may reach as high as 1800 to 2300psi in late model engines

Compared to gasoline-fueled engines, DI

combustion chambers are slower accelerating and in have lower top engine rpm. A 2500-rpm limit in naturally aspirated applications is typical. Higher speeds on turbocharged engines are possible but not practical for several reasons

Lower engine RPM is principally due to the degree of chamber turbulence. DI turbulence is relatively small and a significant amount of time is required to mix the air/fuel and to vapourize the atomized fuel for good combustion. Since there is fewer than 20 degrees of crank rotation to inject, atomize, distribute and vaporize the fuel for combustion, engine speeds are consequently

slower to allow sufficient time for these processes. Typically, injection rates cannot be sustained usually beyond 2500 to 3000rpm without excessive smoking in DI chambers.

Above: Emissions Form Diesel Combustion Are Proportionally Different than A Gasoline Engine

DI’s have a louder sharper fuel knock due to rapid combustion phase and extreme rise in rate of cylinder pressure. (See pilot injection or throttle-pintle injection for improvements to this characteristic)

Other Characteristics of DI Combustion Due to the small idle fuel quantities and low cylinder pressures DI combustion qualities tend to be very poor. Low injection quantities and the “lean” air fuel ratios – as high as 1000:1 produce little heat and pressure. Consider that a single drop of fuel has a volume of 30cubic millimetres. Typical idle injection quantity in a large displacement engine is between 3 and 7 cubic mm. Since heat and pressure accelerate chemical reactions fuel does not burn completely resulting in smoke and excessive soot production. “Turbo slobber” “wet stacking” are phenomenon as a result of excessive idle. Since combustion temperatures are cooler than other engines, less heat is “rejected” to the cooling system. DI chambers do not allow the engine to warm-up and in fact an engine will cool down below operating temperature when idled. Cold cylinders allow pistons to operate in a collapsed – unexpanded. This in turn contributes to slobber because oil can move past piston rings more readily. Several strategies are used to minimize the detrimental effects of prolonged cold idle. Exhaust backpressure governors and regulators cause heat retention in the cylinders and expand piston rings. Half engine mode on Detroit 60’s double injection quantities as it fires alternate cylinders. Increasing engine RPM – fast Idle dash switches or idle shut-down timers are also used.

Rate Shaped

Injection to Minimize Noise & Emissions

Influence of Combustion Chamber Air Motion The addition of turbulence to the incoming air charge is important to mixing the air and fuel in the cylinder to obtain good emission characteristics. In both illustrations of open or direct injection combustion chambers, a swirl is imparted to the air as it enters the chamber. A radial swirl is imparted as air enters the cylinder and a rotational turbulence from the radius bowl of the piston on the piston up-stroke. When the piston compresses the turbulent mass, hundreds of smaller more violent miniature tornadoes are created. Diesel fuel injected into the chamber continues to mix and remix. Correct matching of spray patterns from injector nozzles are required to obtain optimum mixing. Injectors are matched by to the combustion chamber by the manufacturer to ensure good performance and emission compliance. An additional washer installed or not installed at all can affect the power and emission characteristic of a combustion chamber. Adjacent: “Will fit” pistons from an E-7 Mack

Low Turbulence Combustion Chambers A recent trend in combustion chamber design is low swirl or quiescent combustion chambers. Fuel in a high swirl chamber can readily deposit onto the cylinder walls, enter crevice volume, Spray overlap will increase emission level of HC, CO, and particulate. It is also observed that the fuel spray is not evenly mixed in high turbulence chambers, thus having different concentrations of air and fuel. (Stratified fuel charge) The alternative strategy is to minimize combustion chamber turbulence, increase injection pressures and increase the number of orfi in the injector tip

The quality of diesel injection is determined by: Droplet size

Droplet distribution

Spray Penetration

Spray Angle

Pressure Volume Cycle and Combustion Events In A Direct Injection Combustion Chamber After being injected, diesel fuel spray requires time to absorb heat from the compressed air and ignite. It is desirable to have peak cylinder pressure occurring approximately 10 degrees after TDC in a cylinder. Ignition lag (the period between point A & B) is the term used to describe the delay between the start of injection and the ignition of the fuel. Ideally, the smaller the delay period the better. A small delay is 0.001second. 0.003 seconds or more results in a rough running engine and fuel knock since fuel is being injected into the cylinder and not burning. When the fuel begins to ignite more fuel is present in the cylinder for combustion than normally would be. Since the piston has risen further in the cylinder the clearance volume is smaller creatin higher heat

and pressures, which accelerate the combustion process abnormally. (Remember heat and pressure speed up chemical reactions!) These conditions results in a rapid rise in cylinder pressure rather than a more controlled burn once the fuel begins to ignite Ignition lag is influenced by:

·Fuel quality (cetane rating mostly), ·Compression ratio ·Atomization of the fuel. · Pre-Ignition Temperature

Flame Propagation is the period occurs between points B - C on the PV graph. This is a time of uncontrolled burn in a cylinder where fuel injected to the end of point C is combusted

Controlled Burn phase is between point C -D when the remaining fuel is injected. This causes a gradual rise in cylinder pressure after TDC. Some engines will cut off fuel delivery before TDC, at TDC or just after TDC. Peak cylinder pressures will vary from between 1200 and 2300psi

The flame temperature of diesel is ~3900F

After burn is the point from D-E where the remaining fuel is injected where point D the last remaining droplets of fuel are injected. This produces pressure on the piston to keep it moving during its power stroke. The after burn period can affect particulate emission significantly. Too long an after burn phase caused by poor mixing of air and fuel prior to this phase will not allow complete combustion since cylinder pressure and temperatures have decreased due to the large distance of downward piston movement.

Indirect Injection Indirect injection combustion chamber are commonly found in “high-speed” automotive engines as well as refers and other small bore diesel where “gasoline like” performance is desired yet many diesel advantages are retained. The characteristics of these types of chambers include the following. • Uses a pre-combustion chamber formed in the cylinder head. The chamber commonly holds ~ 70% of the cylinder volume. • Uses pintle type injector nozzles with lower injection pressures. 900 - 1800 psi N.O.P. • Higher top engine RPM limit due to higher turbulence resulting in faster mixing and combustion of fuel • Volumetric efficiency can be improved since the valves are larger since they do not compete for space with an injector nozzle. • Cleaner emissions due to better mixing of fuel and air. There is also a longer “residency time” or after burn period. • Better tolerance for poor grades of fuels • The two combustion chamber design creates a quieter sounding engine with less “knock”.

Above: A Ricardo Comet IDI Chamber commonly applied in light duty automotive applications – Ford, GM, Volkswagon, Mitsubishi etc..

Above: Caterpillar IDI Combustion Chamber - Removable Glow-plugs & IDI Chambers Another important characteristic of the IDID chamber is the need for a glow- plug starting aid. This is required since the fuel injector nozzle is a single hole – pintle type that has a low-pressure coarse spray. During starting the chamber is cold and the coarse spray does not develop enough fuel vapour to ignite the fuel to initiate combustion. When the glow-plug is electrically heated to incandescence, the fuel striking the element heats and vaporizes quickly and generates sufficient vapour for combustion. When the chamber is heated by combustion, the walls will provide a sufficiently heated surface to cause vaporization of the fuel for combustion. The glow plug will often stay heated until the engine coolant or oil temperature are sufficient – (“after glow”)

A Burned-out Glow-plug

Above: Pintle nozzle (with rate shaping) Low-pressure, low-cost distributor type pumps used with IDI make this popular with automotive manufacturers

IDI Chamber Disadvantages: • Lower thermal efficiency due to: Large combustion chamber surface area compared to combustion volume. This results in greater heat loss to the coolant and metal surrounding the combustion chamber. For this reason IDI’s will have higher compression ratios to compensate for the heat loss. • Pumping losses because of restriction of pre-cup venturi. (*Pumping Loss the amount of energy expended pulling air into a cylinder and pushing exhaust gases out. A diesel has no throttle plate so pumping losses are less than gasoline or throttled engines. The pre-chamber venturi will increase pumping loss) The poor thermal efficiency combined with the pumping losses causes the engine to operate 10-15% less efficiently than the DI. This requires higher compression ratios to increase compression temperatures and expansion ratios. In turn higher CR’s requires heavier engine parts and and tighter sealing. Higher frictional losses are a consequence of longer strokes and larger bearing surfaces. Losses become greater at higher RPM and fuel economy decreases at higher engine speeds