Diesel Cycle Engines
DIESEL CYCLE ENGINES
The diesel engine is one of the most widely used global powerplants and can be found in almost every conceivable application. From small single-cylinder models to V20 designs, their horsepower can range from as low as 3.73 kW (5 hp) to as high as 46,625 kW (62,500 hp). Some important applications of the diesel engine include
- pickup trucks
- riding lawnmowers
- heavy-duty trucks
- industrial power-generating plants
- oilfield exploration equipment
- road-building equipment (e.g., backhoes, excavators, crawler tractors, graders, and bottom dumps)
- agricultural, logging, and mining equipment
- pleasure craft
- sailboat auxiliary engines
- workboats (e.g., tugs)
- oceangoing merchant ships and passenger liners.
In addition, a wide variety of military equipment, including tanks, armored personnel carriers, HUMVEEs and ships, is powered by diesel engines. The governed speed of diesel engines can range from as low as 85 rpm in large-displacement, slow-speed models, to as high as 5,500–6,000 rpm in smaller automotive type models.
Although today's technologically advanced diesel engine is named after the German Rudolph Diesel, it is a direct result of developmental work that began in the late 1700s when the first internal-combustion engine was constructed. This basic concept was further developed in 1824 by a young French engineer named Sadi Carnot. Other individuals added to this knowledge: Lenoir in 1860 with the first commercial internal-combustion engine; Beau De Rochas in 1862; Otto in cooperation with Langen in 1867; Clerk in 1881; Ackroyd-Stuart in 1890, and finally Diesel in 1892. Since its first practical inception in 1895 by Rudolph Diesel, the diesel cycle engine has been a source of reliable, efficient, long-lasting power.
Both gasoline and diesel engines are available in either a two-stroke- or a four-stroke-cycle design. The fundamental difference between the Otto engine cycle (named after Nikolaus Otto, who developed it in 1876) and the diesel engine cycle involves the conditions of the combustion. In the Otto cycle, the almost instantaneous combustion occurs at a constant volume, before the piston can move much. The pressure goes up greatly during combustion. However, in the diesel cycle, combustion occurs under constant pressure, at least for a time, because the piston moves to increase the volume during the burn to hold the pressure constant.
The four-stroke-cycle internal-combustion engine design is widely employed in both gasoline and diesel engines. The high-speed four-stroke-cycle diesel engine produces superior fuel economy, lower noise factors, and ease of meeting exhaust emissions regulations over its two-stroke-cycle counterpart. In the four-stroke-cycle diesel engine, the concept shown in Figure 1 is used. A total of 720 degrees of crankshaft rotation (two complete revolutions) are required to complete the four piston strokes of intake, compression, power, and exhaust. The actual duration in crankshaft degrees for each stroke is controlled by both the opening and closing of the intake and exhaust valves by the camshaft and will vary among makes and models of engines.
In Figure 1 the engine crankshaft is rotating in a clockwise direction when viewed from the front of the engine. During both the intake and the power strokes, the piston moves down the cylinder, while on both the compression and the exhaust strokes the piston moves up the cylinder. Basically during the intake and exhaust strokes the piston acts as a simple air pump by inducting air and expelling burned exhaust gases from the cylinder. On the compression stroke the upward-moving piston raises the air charge to a pressure typically between 30 and 55 bar (441 and 809 psi) based upon the piston compression ratio and whether the engine is naturally aspirated or employs an exhaust-gas-driven turbocharger to boost the air supply pressure. A net loss in energy (waste heat to the cooling, lubrication, and exhaust systems, and to friction and radiation) occurs during the intake, compression, and exhaust strokes, since only during the expansion or power stroke do we return energy (torque, which is a twisting and a turning force) to rotate the crankshaft.
The diesel engine operates with a much higher compression ratio (CR) than does a gasoline engine, and therefore is manufactured with structurally stronger components capable of handling this design feature. This higher CR results in a much higher cylinder pressure and temperature; therefore a greater expansion rate occurs when the piston is driven down the cylinder on its power stroke than occurs in a gasoline engine. CR is the difference between the volume of air remaining above the piston while at bottom dead center (BDC), versus that at top dead center (TDC). Typically a gasoline engine will operate with CRs between 9 and 10.5:1, while a DI (direct-injected) diesel CR usually varies between 15 and 17:1. IDI (indirect-injected) engine model CRs usually range between 18 and 23:1. These high CRs result in compressed air temperatures prior to the delivery of fuel from the fuel injector, typically in a range between 649°C to 927°C (1,200°F to 1,700°F). This hot air converts the injected fuel from an atomized liquid to a vapor to permit self-ignition (it establishes a flame front) without the need for a spark plug.
DIESEL CONSTANT PRESSURE CYCLE
Diesel's original concept was for his slow-speed engine to operate on a constant-pressure design throughout the power stroke, obtained by continually injecting both compressed air and fuel. To increase the efficiency of the diesel cycle, his first engines used no cooling system, with disastrous results. Later engines, with cooling systems, corrected this part of the problem but resulted in cylinder heat losses accompanied by frictional, radiated, and exhaust heat losses. In addition, although compressed air and fuel were supplied to the cylinder throughout the power stroke, the increasing cylinder volume as the piston moved down on its power stroke was unable to maintain a high enough air temperature and pressure to sustain effective combustion, therefore the air/fuel ratio was not conducive to continued combustion. In today's electronically controlled high-speed diesel engines, fuel is injected for a number of degrees after top dead center (ATDC). This maintains cylinder pressure at a fairly constant level for a given time period, even though there is an increase in clearance volume above the descending piston. Engineers designing and testing engines like to compare the air standard cycles under actual engine performance with corresponding values for highly idealized cycles based on certain simplified assumptions.
The energy used and returned to the engine crankshaft/flywheel is illustrated in Figure 2, showing a PV (pressure-volume) diagram for a turbocharged and direct-injected high-speed heavy-duty four-stroke-cycle diesel engine. This schematic simplifies the internal operation of a piston throughout its four strokes. In Figure 3 we show the actual combustion operating principle in graphic form, with the piston at 90 degrees before TDC and at 90 degrees ATDC.
In Figure 2 you can see that from position 1 to 2 the piston moves down the cylinder on the intake stroke as it is filled with turbocharger boost air higher than atmospheric pressure, as indicated in line Pl. Depending on the valve timing, actual inlet valve closure will control the degree of trapped cylinder air pressure. In this example, compression starts at position 2, as the piston moves up the cylinder. Fuel is injected at a number of degrees BTDC (before top dead center), and there is both a pressure and a temperature rise as the fuel starts to burn from positions 3 to 4. As the piston moves away from TDC on its power stroke, positions 4 to 5, the continuous injection of fuel, provide a constant pressure for a short number of crankshaft degrees. From positions 5 to 6, the piston is driven downward by the pressure of the expanding gases. The point at which the exhaust valves open BBDC (before bottom dead center) depends on the make and model of the engine used. The work represented by area 6–7–2 is available to the hot end of the turbocharger (turbine wheel) from the hot, pressurized exhaust gases. Line PA indicates atmospheric pressure along points 9–10–11. The exhaust manifold pressure is shown as line PB and the exhaust gas blow-down energy is represented by points 6–9–10. The exhaust process from the engine cylinder is shown among points 6, 13 and 12, where 6 through 13 is the blow-down period when the exhaust valves open and the high-pressure gases expand into and through the exhaust manifold. From points 13 to 12 the piston moves from BDC to TDC, displacing most of the exhaust gas out of the cylinder. Therefore the potential work of the exhaust gases in this turbocharged engine is represented by the crosshatched areas identified as points 10–11–12–13. The maximum energy to drive the turbine of the turbocharger is that shown in the area identified as points 6–9–10 and 10–11–12 and 13. Ideally during points 6–9–10, if both the cylinder pressure and the turbocharger inlet pressure could both be maintained at equal levels before the piston moves upward from BDC on its exhaust stroke, a system close to ideal would be created when using a pulse turbocharger system.
The higher CR and the fact that diesel fuel contains a higher heat content per gallon or liter (approximately 11%) than does gasoline are two of the reasons why the diesel engine produces better fuel economy and higher torque at the crankshaft and flywheel. Further fuel economy improvements can be attributed to the fact that in the diesel engine, the throttle pedal is used to control the cylinder fueling rate directly. In a gasoline engine, however, manual operation of the throttle (gas pedal) directly controls the volume of air entering the engine intake manifold by restricting the size of the opening.
Diesel engines, however, operate on an unrestricted air flow at all speeds and loads to provide the cylinders with an excess air charge. This results in a very lean air/fuel ratio of approximately 90:1 to 100:1 or higher at an idle speed. At the engine's rated speed (full load maximum power output) the air/fuel ratio will drop to 20:1 to 25:1 but still provide an excess air factor here of 10 to 20 percent. This excess air supply lowers the average specific heat of the cylinder gases, which in turn increases the indicated work obtained from a given amount of fuel. Compare this to most gasoline electronically controlled engines, where the TBI (throttle body injection) or MPFI (multiport fuel injection) system is designed to operate at a stoichiometric or 14.7:1 air/fuel ratio.
TWO-STROKE-CYCLE DIESEL ENGINE
Figure 4 illustrates the basic operational concept for a two-stroke cycle vee-configuration diesel engine where a power stroke is created every 360 degrees of crankshaft rotation versus the 720 degrees needed in the four-stroke-cycle design. The two-stroke design eliminates the necessity for individual intake and exhaust strokes required in a four-stroke cycle engine; therefore it becomes necessary in the two-stroke-cycle engine to employ a gear-driven blower to supply a large supply of low-pressure fresh air for:
- combustion of the injected fuel;
- cooling (approximately 30% of the engine cooling is performedby air flow, while 70% is performed by coolant flow within the engine radiator or heat exchanger);
- scavenging of exhaust gases from the cylinders;
- positive crankcase ventilation.
In the two-stroke-cycle diesel engine the cylinder-head-located poppet valves are to permit scavenging of the exhaust gases. Fresh air for the four functions listed above is force-fed through a series of ports located midway around the cylinder liner. Each liner receives its air supply from an "air box" cast within the engine block. On high-speed diesel engines this air pressure varies between 27.6 kPa and 48.3 kPa (4 and 7 psi) higher than atmospheric throughout the engine speed range. When an exhaust-driven turbocharger is used in conjunction with the gear-driven blower, air-box boost pressures at engine full load typically range between 172 and 207 kPa (25 and 30 psi) above atmospheric.
With reference to Figure 4, each piston down-stroke provides power, while each upstroke provides compression of the blower/turbo-supplied cylinder air. The actual number of degrees of both of these strokes will vary based on the specific engine make and model, and its year of manufacture needed to comply with U.S. Environmental Protection Agency (EPA) exhaust emission limits.
Note in Figure 4 that in a high-speed engine, the power stroke begins at TDC and ends at approximately 90 to 92 degrees ATDC (after top dead center), when the exhaust poppet valves start to open by camshaft action. This allows the pressurized exhaust gases to start flowing from the cylinder through the open exhaust valves. As the piston continues moving down the cylinder, the exhaust gas pressure decreases. At approximately 59 degrees BBDC (before bottom dead center), the piston begins to uncover the cylinder liner ports, permitting the now higher air-box pressure (ABP) to enter the cylinder; this is the start of the "scavenging" stroke. Since the ABP is now higher than the exhaust gas pressure, positive displacement of the exhaust gases out of the cylinder takes place. The scavenging process lasts for approximately 118 degrees of crankshaft rotation (59 degrees BBDC and 59 degrees ABDC (after bottom dead center). It is during this 118-degree period that scavenging; cooling of the piston, liner, valves, and cylinder head; and inducting fresh air for combustion purposes occur.
During the piston upstroke the exhaust valves do not close until after the cylinder liner ports have been covered at approximately 59 degrees ABDC by the upward-moving piston. Typically the exhaust valves close between 3 and 5 degrees after port closure. The piston is now on its compression stroke. The start of fuel injection at all speeds is variable, based on the year of manufacture, the make and model of the engine, and whether it is mechanically or electronically controlled. The fuel injection duration typically lasts for 10 to 14 degrees at an idle speed between 500 and 700 rpm beginning at BTDC and ending just at TDC, or a few degrees ATDC based on the engine make and model and its year of manufacture.
DIESEL COMBUSTION TYPES
The majority of existing diesel engines now in use operate on what is commonly referred to as a direct-injection (DI) design (see Figure 5). This means that the high-pressure injected fuel (as high as 30,000 psi or 207 MPa) enters directly into the combustion chamber formed by the shape of the piston crown. In the indirect-injection (IDI) system the injected fuel is sprayed into a small antechamber within the cylinder head. Combustion begins in this small chamber and forces its way into the main chamber, where it consumes the remaining air required for additional combustion. IDI engine designs require use of an electrical glow plug to initiate satisfactory combustion, something that is not required in a DI engine. Use of a glow plug allows the IDI engine to burn a rougher grade of fuel than the DI engine. Fuel is injected at a lower pressure in the IDI; in addition, the larger combustion surface area of the IDI engine creates greater heat losses. This results in the IDI engine consuming approximately 15 percent more fuel than an equivalent horsepower (kW) rated DI engine. Additionally, using a rougher grade of diesel fuel results in higher exhaust emissions than from a DI engine using high-quality, low-sulfur fuel.
Rudolph Diesel's original intent was to produce a low-heat-rejection internal-combustion engine without the need for a cooling system. He believed that this would provide less heat losses from the combustion process and provide him with a superior heat, or thermally efficient (TE), design concept. To his chagrin, however, he found that this was not a feasible option when his first several test engines failed to perform to plan.
Basic physics involving friction and heat losses prevents the construction of a perfect internal-combustion engine. If friction between moving components could be eliminated, the mechanical efficiency (ME) of the engine would improve. Similarly, if we could eliminate heat losses from the combustion process, we could improve the TE of the combustion process. If no friction or heat losses existed we could design a perfect or ideal engine—one that could provide closer to 100 percent efficiency. A simplified way to consider TE is that for every dollar or hundred cents of fuel consumed by the internal-combustion engine, how much is returned as usable power? Therefore the TE is a comparison of the actual ratio of useful work performed in the engine versus the total energy content of the fuel consumed. Typically gasoline engines today return twenty-eight to thirty-six cents on the dollar. The diesel engine returns approximately forty to forty-three cents on the dollar. It is a measure of how efficiently an internal-combustion engine uses the heat released into the combustion chamber from the fuel to produce mechanical power. Based on the specific make and model of the engine, the cooling and exhaust systems typically account for heat losses of about 23 to 27 percent each; friction losses can range between 7 and 9 percent; while radiated heat from the engine accounts for 3 and 5 percent. Therefore this combination will generally account for a 57 percent heat loss, resulting in a TE of approximately 43 percent in current electronically controlled high-speed diesel engine models. However, many stationary diesel power plants that recapture waste exhaust heat for cogeneration purposes can return TEs in the mid-50-percent and higher ranges. New mechanical and electronic design concepts plus the adoption of ceramic components are just some of the new technologies being adopted not only to improve thermal efficiency but also to drastically reduce exhaust emissions into the atmosphere.
The diesel engine without the benefit of a spark plug does not generate instantaneous combustion, as occurs within the gasoline engine. Instead, the diesel cycle relies on the high-pressure atomized-injected fuel mixing with the hot compressed air to cause it to vaporize. Once this vaporization occurs, the air/fuel mixture generates a flame front to initiate combustion. This concept creates what is known as "ignition delay" and is one of the characteristics that gives the diesel cycle its unique pinging noise. The longer the ignition delay, the louder the combustion noise (hard combustion) due to the larger volume of injected fuel that collects within the combustion chamber prior to actual ignition. The start of ignition includes the fuel injected prior to this phase and is known as the "premixed flame."
Once the fuel ignites, the remaining fuel being injected has no ignition delay, since it is being sprayed directly into an established flame front. Under full-load conditions, peak cylinder pressures can average between 1,800 and 2,300 psi (12,411 and 15,859 kPa). These tremendous pressures produce the power within the diesel engine during the power stroke, resulting in a higher overall BMEP (brake mean effective pressure—the average pressure exerted on the piston throughout the power stroke), versus the much lower values in a gasoline engine and the higher TE levels of the diesel.
DIESEL EXHAUST EMISSIONS
All internal-combustion engines, due to their inherent design characteristics, are unable to burn the injected fuel to completion. The make, model, year of manufacture, cylinder displacement, speed, and load all affect the percentage of emissions emitted into the atmosphere and the air we breathe. Therefore, major research and development is a continuing effort to clean up the type and quantity of pollutants. The EPA is a government agency charged with setting the limits on all industrial and internal-combustion-engine limits. The European Economic Community as well as Asian and other countries have similar agencies tasked within these same parameters.
Major exhaust emissions from internal-combustion engines targeted by the EPA can be categorized into the following areas:
- Carbon dioxide (CO2), although nonpoisonous, does contribute to "global warming." Complete combustion in an internal combustion engine produces CO2 and water.
- Carbon monoxide (CO) is a colurless, odorless, and tasteless gas. Inhalation of as little as 0.3 percent by volume can cause death within thirty minutes. The exhaust gas from spark ignition engines at an idle speed has a high CO content. For this reason NEVER allow an engine to run in an enclosed space such as a closed garage.
- Oxides of nitrogen (NOx) have two classes. Nitrogen monoxide (NO) is a colorless, odorless, and tasteless gas that is rapidly converted into nitrogen dioxide (NO2) in the presence of oxygen. NO2 is a yellowish-to-reddish-brown poisonous gas with a penetrating odor that can destroy lung tissue. NO and NOx are customarily treated together and referred to as oxides of nitrogen.
- Hydrocarbons of many different types are present in exhaust gas. In the presence of nitrogen oxide and sunlight, they form oxidants that irritate the mucous membranes. Some hydrocarbons are considered to be carcinogenic. Incomplete combustion produces unburned hydrocarbons.
- Particulate matter, in accordance with U.S. legislation, includes all substances (with the exception of unbound water) that under controlled conditions are present as solids (ash, carbon) or liquids in exhaust gases.
Diesel engines, due to the combustion processes described herein, tend to have a rougher time meeting some of the specific exhaust emissions standards. The primary cause of combustion noise and the generation of oxides of nitrogen in the diesel engine can be traced to that portion of the combusted fuel that burns as a very rapid premixed flame. On the other hand, the slower-burning diffusion flame (fuel-injected after the start of ignition) is the primary cause of soot and unburned hydrocarbons.
At this time it is not possible to produce a totally soot-free diesel engine because heterogeneous combustion always produces soot. Diesel engine operation, due to local concentrations of overly rich mixtures in the diffusion flame, leads to an increase in the emission of black smoke to a moderate extent even with moderate excess air. The relatively low exhaust gas temperatures of diesel engines create a problem for effective catalytic emission control of hydrocarbons, particularly in light-duty diesel engines.
Reduction of exhaust emissions is being tackled in two ways by engineers, including precombustion and postcombustion technology. One of the most effective methods now being researched and adopted includes use of synthetic fuel made from natural gas. This fuel is crystal clear, and just like water, it has no aromatics, contains no sulfur or heavy metals, and when used with a postcombustion device such as a catalytic converter any remaining NOx or other emissions can be drastically reduced. Estimates currently place the cost of this fuel at $1.50 per gallon, with availability in 2004 to meet the next round of stiff EPA exhaust emission standards.
Some precombustion technology involves improvements in internal engine hardware components, various engine sensors, and electronically controlled common-rail fuel injection equipment. Other systems now on test incorporate the addition of a small quantity of a reducing agent such as urea (sometimes called carbamide) injected into the combustion chamber, resulting in a chemical reaction that releases ammonia. This in turn converts the exhaust gases into nontoxic levels of nitrogen and water.
The second method used to reduce exhaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO2 and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exhaust, and reduce by more than 90 percent emissions of CO and hydrocarbons.
EPA diesel exhaust emissions limits for 1998 on-highway diesel truck and bus engines in g/bhp-hr (grams/brake horsepower/hour) when using existing 0.05 percent low-sulfur diesel fuel were: hydrocarbons, 1.3; CO, 15.5; NOx, 4.0; and particulate matters, 0.1. Regulations due to come into effect beginning with the 2004 model year represent approximately a 50 percent reduction in emissions of NOx and particulate matters, as well as reductions in hydrocarbons.
DIESEL AUTOMOTIVE CONSUMER RELUCTANCE
Despite the superior fuel economy of the diesel engine and its longer life to overhaul versus that for most of its equivalent gasoline counterparts, the general automotive consumer has preferred the choice of the gasoline engine in passenger cars and light trucks. However, within the European Community, the better fuel economy of the diesel accounts for up to 35 to 40 percent of all vehicle sales. In North America, diesel pickup trucks from all of the domestic manufacturers are popular options. To capture a greater percentage of the vehicle market, diesel engines require some technological improvements to bring their overall performance closer to that of the gasoline engine.
The following are the nine basic advantages of gasoline over diesel:
- Quieter operation (no ignition delay or diesel knock; lower peak cylinder pressures and temperatures).
- Easier starting, particularly in cold-weather operation (gasoline vaporizes at a much lower temperature than does diesel fuel; spark plugs provide instant combustion).
- Less unpleasant odor, particularly in the exhaust gases.
- Tends to burn visually cleaner at the exhaust tailpipe, since it operates in a closed-loop electronic mode (oxygen sensors interacting with the powertrain control module) to maintain an ideal air/fuel ratio of 14.7:1.
- Quicker acceleration (no ignition delay) at lower engine speeds.
- Can operate at higher speeds (rpm). Less inertia forces due to lighter components.
- Good fuel economy at steady-state highway cruising speeds.
- Lower weight, resulting in a higher power-to-weight density.
- Generally lower production costs.
The following are the four basic advantages of diesel over gasoline:
- Greater mileage between engine overhaul/ repair (more robust).
- Superior fuel economy (more thermally efficient) particularly at low speeds due to lack of restriction of the air flow; air flow restriction occurs in a gasoline engine through the throttling action of the gas pedal.
- Lower carbon monoxide levels.
- Higher crankshaft torque-producing capability.
DIESEL TECHNOLOGICAL ADVANCES
Since the 1985 model year, many heavy-duty high-speed diesel engines have been equipped with turbocharged aftercooled engines using electronic fuel injection controls. By the 1990 year all major high-speed engine original equipment manufacturers (OEMs) in North America employed electronic controls, since adopted by other diesel engine OEMs. Metallurgical advances have provided lighter but stronger engine components, and the use of plastics and fiberglass and aluminum alloys has increased in many external engine components
Four-valve cylinder heads, overhead camshafts, ceramic turbocharger components, crossflow cylinder heads, two-piece cross-head pistons, electronically controlled injectors, and hydraulically actuated electronically controlled unit injectors that do not require a pushrod and rocker arm assembly are all in use on existing engines. Future rockerless valve control engines, nonferrous piston and liner components, and turbocompounding will all improve the thermal efficiency of future engines.
Variable valve timing similar to that now in use in gasoline engines will become more common on all internal-combustion engines. Future valveless engines might employ rotating hollow-type shafts in place of the long-used poppet valves, or an electric solenoid will be used to operate both intake and exhaust valves, once again reducing valve-train frictional losses to improve overall thermal efficiency. Turbocompounding is a process whereby the hot, pressurized exhaust gases, after driving the turbocharger, will be directed to a large expansion turbine geared to the engine crankshaft to return additional energy, which would otherwise be wasted by flowing out of the exhaust system. The result will be a substantial increase in thermal efficiency. Turbocompounding is not yet available in a full-scale production engine, but look for this feature on future engines. Low-flow cooling and lube systems that have been in use for some time have reduced parasitic losses to further improve fuel economy. Future cooling systems will employ ceramic components, permitting the engine to run at a higher coolant temperature and providing a further increase in thermal efficiency.
Robert N. Brady
See also: Automobile Performance; Carnot, Nicolas Leonard Sadi; Combustion; Diesel Fuel; Diesel, Rudolph; Engines; Gasoline and Additives; Gasoline Engines; Government Agencies; Otto, Nikolaus August; Thermodynamics.
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