Diesel Power

Discussion in 'Trucks and Trailers' started by nosparkplugs, Mar 2, 2008.

  1. nosparkplugs

    nosparkplugs LawnSite Gold Member
    Messages: 3,444

    many polled on this site would choose a diesel over gasoline just don't want to pay for it, or justify a gasburner instead; because me employee's will be driving it (no respect) on and on. Companies are switching to diesel power, regardless of who is driving the vehicle, Cost of ownerships is lower, and is becoming the "green solution" of the Lawn care industry, with Biodiesel prices dropping below Petro diesel. Truegreen, and many other companies are now switching their ZTR mower fleets over to diesel, and using B20 blends. Although we may not be as big, or have the financial ability, we must start somewhere. I get many a compliment for my diesels, customers are aware of the Biodiesel explosion,,and gas guzzling V-8 will be replaced by a hybrid gas/electric truck motor. So in time you gas lovers will be forced into another engine type.

    Diesel Power is the Future?

    When working on his calculations, Rudolf Diesel theorized that higher compression leads to higher efficiency and more power. This happens because when the piston squeezes air with the cylinder, the air becomes concentrated. Diesel fuel has a high energy content, so the likelihood of diesel reacting with the concentrated air is greater. Another way to think of it is when air molecules are packed so close together, fuel has a better chance of reacting with as many oxygen molecules as possible. Rudolf turned out to be right -- a gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1.

    In mechanical terms, the internal construction of a diesel engine is similar to its gasoline counterpart—components such as pistons, connecting rods and a crankshaft are present in both. Like a gasoline engine, a diesel engine may operate on a four-stroke cycle (similar to the gasoline unit's Otto cycle), or a two-stroke cycle, albeit with significant dissimilarity to the gasoline equivalent. In both cases, the principal differences lie in the handling of air and fuel, and the method of ignition.
    A diesel engine relies upon compression ignition to burn its fuel, instead of the spark plug used in a gasoline engine. If air is compressed to a high degree, its temperature will increase to a point where fuel will burn upon contact. This principle is used in both four-stroke and two-stroke diesel engines to produce power.
    Unlike a gasoline engine, which draws an air/fuel mixture into the cylinder during the intake stroke, the diesel aspirates air alone. Following intake, the cylinder is sealed and the air charge is highly compressed to heat it to the temperature required for ignition. Whereas a gasoline engine's compression ratio is rarely greater than 11:1 to avoid damaging preignition, a diesel's compression ratio is usually between 16:1 and 25:1. This extremely high level of compression causes the air temperature to increase to 700 to 900 degrees Celsius (1300 to 1650 degrees Fahrenheit). If a piece of steel were to be heated to that level it would glow cherry red.
    As the piston approaches top dead center (TDC), fuel oil is injected into the cylinder at high pressure, causing the fuel charge to be atomized. Owing to the high air temperature in the cylinder, ignition instantly occurs, causing a rapid and considerable increase in cylinder temperature and pressure (generating the characteristic Diesel "knock"). The piston is driven downward with great force, pushing on the connecting rod and turning the crankshaft.
    When the piston nears bottom dead center the spent combustion gases are expelled from the cylinder to prepare for the next cycle. In many cases, the exhaust gases will be used to drive a turbocharger, which will increase the volume of the intake air charge, resulting in cleaner combustion and greater efficiency.

    Power and fuel economy
    Diesel engines are more efficient than gasoline (petrol) engines of the same power, resulting in lower fuel consumption. A common margin is 40% more miles per gallon for an efficient turbodiesel. For example, the current model _koda Octavia, using Volkswagen Group engines, has a combined Euro rating of 38 miles per US gallon (6.2 L/100 km) for the 102 bhp (76 kW) petrol engine and 54 mpg (4.4 L/100 km) for the 105 bhp (78 kW) diesel engine. However, such a comparison doesn't take into account that diesel fuel is denser and contains about 15% more energy by volume. Although the calorific value of the fuel is slightly lower at 45.3 MJ/kg (megajoules per kilogram) than gasoline at 45.8 MJ/kg, liquid diesel fuel is significantly denser than liquid gasoline. When this is taken into account, diesel fuel has a higher energy density than petrol; this volumetric measure is the main concern of many people as diesel fuel is sold by volume, not weight, and must be transported and stored in tanks of fixed size.

    Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules (147,000 BTU), while 1 gallon of gasoline contains 132x106 joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines.

    Adjusting the numbers to account for the energy density of diesel fuel, one finds the overall energy efficiency of the aforementioned paragraph is still about 20% greater for the diesel version, despite the weight penalty of the diesel engine. When comparing engines of relatively low power for the vehicle's weight (such as the 75 hp VW Golf), the diesel's overall energy efficiency advantage is reduced further but still between 10 and 15 percent.

    While higher compression ratio is helpful in raising efficiency, diesel engines are much more economical than gasoline (petrol) engines when at low power and at engine idle. Unlike the petrol engine, diesels lack a butterfly valve (throttle) in the inlet system, which closes at idle. This creates parasitic drag on the incoming air, reducing the efficiency of petrol/gasoline engines at idle. Due to their lower heat losses, diesel engines have a lower risk of gradually overheating if left idling for long periods of time. In many applications, such as marine, agriculture, and railways, diesels are left idling unattended for many hours or sometimes days. These advantages are especially attractive in locomotives (see dieselization).
    Naturally aspirated diesel engines are heavier than gasoline engines of the same power for two reasons. The first is that it takes a larger displacement diesel engine to produce the same power as a gasoline engine. This is essentially because the diesel must operate at lower engine speeds.[5] Diesel fuel is injected just before ignition, leaving the fuel little time to reach all the oxygen in the cylinder. In the gasoline engine, air and fuel are mixed for the entire compression stroke, ensuring complete mixing even at higher engine speeds. The second reason for the greater weight of a diesel engine is it must be stronger to withstand the higher combustion pressures needed for ignition, and the shock loading from the detonation of the ignition mixture. As a result, the reciprocating mass (the piston and connecting rod), and the resultant forces to accelerate and to decelerate these masses, are substantially higher the heavier, the bigger and the stronger the part, and the laws of diminishing returns of component strength, mass of component and inertia — all come into play to create a balance of offsets, of optimal mean power output, weight and durability.
    Yet it is this same build quality that has allowed some enthusiasts to acquire significant power increases with turbocharged engines through fairly simple and inexpensive modifications. A gasoline engine of similar size cannot put out a comparable power increase without extensive alterations because the stock components would not be able to withstand the higher stresses placed upon them. Since a diesel engine is already built to withstand higher levels of stress, it makes an ideal candidate for performance tuning with little expense. However, it should be said that any modification that raises the amount of fuel and air put through a diesel engine will increase its operating temperature which will reduce its life and increase service requirements. These are issues with newer, lighter, high performance diesel engines which aren't "overbuilt" to the degree of older engines and are being pushed to provide greater power in smaller engines.
    The addition of a turbocharger or supercharger to the engine greatly assists in increasing fuel economy and power output, mitigating the fuel-air intake speed limit mentioned above for a given engine displacement. Boost pressures can be higher on diesels than gasoline engines, due to the latter's susceptibility to knock, and the higher compression ratio allows a diesel engine to be more efficient than a comparable spark ignition engine. Because the burned gases are expanded further in a diesel engine cylinder, the exhaust gas is cooler, meaning turbochargers require less cooling, and can be more reliable, than on spark-ignition engines.
    The increased fuel economy of the diesel engine over the gasoline engine means that the diesel produces less carbon dioxide (CO2) per unit distance. Recently, advances in production and changes in the political climate have increased the availability and awareness of biodiesel, an alternative to petroleum-derived diesel fuel with a much lower net-sum emission of CO2, due to the absorption of CO2 by plants used to produce the fuel.
    The two main factors that held diesel engine back in private vehicles until quite recently were their low power outputs and high noise levels, characterised by knock or clatter, especially at low speeds and when cold. This noise is caused by "piston slap", the sudden ignition of the diesel fuel when injected into the combustion chamber slamming the cold-contracted piston into the cylinder wall. The tolerances between the piston and cylinder wall are greater at cold temperatures to allow expansion at higher temperatures. A combination of improved mechanical technology (such as two-stage injectors which fire a short "pilot charge" of fuel into the cylinder to warm the combustion chamber before delivering the main fuel charge) and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures) have partially mitigated these problems in the latest generation of common-rail designs. Poor power and narrow torque bands have been helped by the use of turbochargers and intercoolers.

    Power and torque
    For commercial uses requiring towing, load carrying and other tasks, diesel engines tend to have better torque characteristics. Diesel engines tend to have their torque peak quite low in their speed range (usually between 1600 – 2000 rpm for a small-capacity unit, lower for a larger engine used in a truck). This provides smoother control over heavy loads when starting from rest, and, crucially, allows the diesel engine to be given higher loads at low speeds than a petrol engine, making them much more economical for these applications. This characteristic is not so desirable in private cars, so most modern diesels used in such vehicles use electronic control, variable geometry turbochargers and shorter piston strokes to achieve a wider spread of torque over the engine's speed range, typically peaking at around 2500 – 3000 rpm.

    The lack of an electrical ignition system greatly improves the reliability. The high durability of a diesel engine is also due to its overbuilt nature (see above) as well as the diesel's combustion cycle, which creates less-violent changes in pressure when compared to a spark-ignition engine, a benefit that is magnified by the lower rotating speeds in diesels. Diesel fuel is a better lubricant than gasoline so is less harmful to the oil film on piston rings and cylinder bores; it is routine for diesel engines to cover 250,000 miles (400 000 km) or more without a rebuild.
    Unfortunately, due to the greater compression force required and the increased weight of the stronger components, starting a diesel engine is a harder task. More torque is required to push the engine through compression.
    Either an electrical starter or an air start system is used to start the engine turning. On large engines, pre-lubrication and slow turning of an engine, as well as heating, are required to minimize the amount of engine damage during initial start-up and running. Some smaller military diesels can be started with an explosive cartridge, called a Coffman starter, which provides the extra power required to get the machine turning. In the past, Caterpillar and John Deere used a small gasoline pony motor in their tractors to start the primary diesel motor. The pony motor heated the diesel to aid in ignition and utilized a small clutch and transmission to actually spin up the diesel engine. Even more unusual was an International Harvester design in which the diesel motor had its own carburetor and ignition system, and started on gasoline. Once warmed up, the operator moved two levers to switch the motor to diesel operation, and work could begin. These engines had very complex cylinder heads, with their own gasoline combustion chambers, and in general were vulnerable to expensive damage if special care was not taken (especially in letting the engine cool before turning it off).
    As mentioned above, diesel engines tend to have more torque at lower engine speeds than gasoline engines. However, diesel engines tend to have a narrower power band than gasoline engines. Naturally-aspirated diesels tend to lack power and torque at the top of their speed range. This narrow band is a reason why a vehicle such as a truck may have a gearbox with as many as 18 or more gears, to allow the engine's power to be used effectively at all speeds. Turbochargers tend to improve power at high engine speeds, superchargers do the same at lower speeds, and variable geometry turbochargers improve the engine's performance equally (or make the torque curve flatter).

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