Locking differential

A locking differential, differential lock, diff lock or locker is a variation on the standard automotive differential. A locking differential may provide increased traction compared to a standard, or "open" differential by restricting each of the two wheels on an axle to the same rotational speed without regard to available traction or differences in resistance seen at each wheel.

A locking differential is designed to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction (or lack thereof) available to either wheel individually.

When the differential is unlocked (open differential), it allows each wheel to rotate at different speeds (such as when negotiating a turn), thus avoiding tire scuffing. An open (or unlocked) differential always provides the same torque (rotational force) to each of the two wheels, on that axle. So although the wheels can rotate at different speeds, they apply the same rotational force, even if one is entirely stationary, and the other spinning. (Equal torque, unequal rotational speed).

By contrast, a locked differential forces both left and right wheels on the same axle to rotate at the same speed under nearly all circumstances, without regard to tractional differences seen at either wheel. Therefore, each wheel can apply as much rotational force as the traction under it will allow, and the torques on each side-shaft will be unequal. (Unequal torque, equal rotational speeds). Exceptions apply to automatic lockers, discussed below.

A locked differential can provide a significant traction advantage over an open differential, but only when the traction under each wheel differs significantly.

All the above comments apply to central differentials as well as to those in each axle: full-time four-wheel-drive (more accurately as "All Wheel Drive") vehicles have three differentials, one in each axle, and a central one between the front and rear axles (transfer case).

Diesel Oxidation Catalyst

For compression-ignition (i.e., diesel engines), the most commonly used catalytic converter is the Diesel Oxidation Catalyst (DOC). This catalyst uses O₂ (oxygen) in the exhaust gas stream to convert CO (carbon monoxide) to CO₂ (carbon dioxide) and HC (hydrocarbons) to H₂O (water) and CO₂. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping to reduce visible particulates (soot). These catalysts are not active for NO_x reduction because any reductant present would react first with the high concentration of O₂ in diesel exhaust gas.

Reduction in NO_x emissions from compression-ignition engines has previously been addressed by the addition of exhaust gas to incoming air charge, known as exhaust gas recirculation (EGR). In 2010, most light-duty diesel manufacturers in the U.S. added catalytic systems to their vehicles to meet new federal emissions requirements. There are two techniques that have been developed for the catalytic reduction of NO_x emissions under lean exhaust conditions - selective catalytic reduction (SCR) and the lean NO_x trap or NOx adsorber. Instead of precious metal-containing NOx adsorbers, most manufacturers selected base-metal SCR systems that use a reagent such as ammonia to reduce the NO_x into nitrogen. Ammonia is supplied to the catalyst system by the injection of urea into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. One trademark product of urea solution, also referred to as Diesel Exhaust Fluid (DEF), is AdBlue.

Diesel exhaust contains relatively high levels of particulate matter (soot), consisting in large part of elemental carbon. Catalytic converters cannot clean up elemental carbon, though they do remove up to 90 percent of the soluble organic fraction, so particulates are cleaned up by a soot trap or diesel particulate filter (DPF). Historically, a DPF consists of a Cordierite or Silicon Carbide substrate with a geometry that forces the exhaust flow through the substrate walls, leaving behind trapped soot particles. Contemporary DPFs can be manufactured from a variety of rare metals that provide superior performance (at a greater expense). As the amount of soot trapped on the DPF increases, so does the back pressure in the exhaust system. Periodic regenerations (high temperature excursions) are required to initiate combustion of the trapped soot and thereby reducing the exhaust back pressure. The amount of soot loaded on the DPF prior to regeneration may also be limited to prevent extreme exotherms from damaging the trap during regeneration. In the U.S., all on-road light, medium and heavy-duty vehicles powered by diesel and built after 1 January 2007, must meet diesel particulate emission limits that means they effectively have to be equipped with a 2-Way catalytic converter and a diesel particulate filter. Note that this applies only to the diesel engine used in the vehicle. As long as the engine was manufactured before 1 January 2007, the vehicle is not required to have the DPF system. This led to an inventory runup by engine manufacturers in late 2006 so they could continue selling pre-DPF vehicles well into 2007. During the re-generation cycle, most systems require the engine to consume several gallons of fuel in a relatively short amount of time in order to generate the high temperatures necessary for the cycle to complete. This has been shown to adversely affect the overall fuel economy of vehicles equipped with DPF systems, especially in vehicles that are driven mostly in city conditions where frequent acceleration requires a larger amount of fuel to be burned and therefore more soot to collect in the exhaust system.

Types of Catalytic converter

Two-way

A two-way (or "oxidation") catalytic converter has two simultaneous tasks:

1. Oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ → 2CO₂
2. Oxidation of hydrocarbons (unburnt and partially burnt fuel) to carbon dioxide and water: C_xH_2x+2 + [(3x+1)/2] O₂ → xCO₂ + (x+1) H₂O (a combustion reaction)

This type of catalytic converter is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on gasoline engines in American- and Canadian-market automobiles until 1981. Because of their inability to control oxides of nitrogen, they were superseded by three-way converters.

Three-way

Three-way catalytic converters (TWC) have the additional advantage of controlling the emission of nitrogen oxides (NO_x), in particular nitrous oxide, a greenhouse gas over three hundred times more potent than carbon dioxide, a precursor to acid rain and currently the most ozone-depleting substance. Technological improvements including three-way catalytic converters have led to motor vehicle nitrous oxide emissions in the US falling to 8.2% of anthropogenic nitrous oxide emissions in 2008, from a high of 17.77% in 1998.

Since 1981, "three-way" (oxidation-reduction) catalytic converters have been used in vehicle emission control systems in the United States and Canada; many other countries have also adopted stringent vehicle emission regulations that in effect require three-way converters on gasoline-powered vehicles. The reduction and oxidation catalysts are typically contained in a common housing, however in some instances they may be housed separately. A three-way catalytic converter has three simultaneous tasks:

1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NO_x → xO₂ + N₂
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ → 2CO₂
3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: C_xH_2x+2 + [(3x+1)/2]O₂ → xCO₂ + (x+1)H₂O.

These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point. This point is between 14.6 and 14.8 parts air to 1 part fuel, by weight, for gasoline. The ratio for Autogas (or liquefied petroleum gas (LPG)), natural gas and ethanol fuels is each slightly different, requiring modified fuel system settings when using those fuels. In general, engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedbackfuel injection system using one or more oxygen sensors, though early in the deployment of three-way converters, carburetors equipped for feedback mixture control were used.

Three-way catalysts are effective when the engine is operated within a narrow band of air-fuel ratios near stoichiometry, such that the exhaust gas oscillates between rich (excess fuel) and lean (excess oxygen) conditions. However, conversion efficiency falls very rapidly when the engine is operated outside of that band of air-fuel ratios. Under lean engine operation, there is excess oxygen and the reduction of NO_x is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, thus only stored oxygen is available for the oxidation function. Closed-loop control systems are necessary because of the conflicting requirements for effective NO_x reduction and HC oxidation. The control system must prevent the NO_x reduction catalyst from becoming fully oxidized, yet replenish the oxygen storage material to maintain its function as an oxidation catalyst.

Three-way catalytic converters can store oxygen from the exhaust gas stream, usually when the air–fuel ratio goes lean. When sufficient oxygen is not available from the exhaust stream, the stored oxygen is released and consumed (see cerium(IV) oxide). A lack of sufficient oxygen occurs either when oxygen derived from NO_x reduction is unavailable or when certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to supply oxygen.

Supersession of carburetors

In the 1970s and 1980s in the US and Japan, the respective federal governments imposed increasingly strict exhaust emission regulations. During that time period, the vast majority of gasoline-fueled automobile and light truck engines did not use fuel injection. To comply with the new regulations, automobile manufacturers often made extensive and complex modifications to the engine carburetor(s). While a simple carburetor system is cheaper to manufacture than a fuel injection system, the more complex carburetor systems installed on many engines in the 1970s were much more costly than the earlier simple carburetors. To more easily comply with emissions regulations, automobile manufacturers began installing fuel injection systems in more gasoline engines during the late 1970s.

The open loop fuel injection systems had already improved cylinder-to-cylinder fuel distribution and engine operation over a wide temperature range, but did not offer further scope to sufficient control fuel/air mixtures, in order to further reduce exhaust emissions. Later Closed loop fuel injection systems improved the air/fuel mixture control with an exhaust gas oxygen sensor and began incorporating a catalytic converter to further reduce exhaust emissions.

Fuel injection was phased in through the latter 1970s and 80s at an accelerating rate, with the German, French, and U.S. markets leading and the UK and Commonwealth markets lagging somewhat. Since the early 1990s, almost all gasoline passenger cars sold in first world markets are equipped with electronic fuel injection (EFI). The carburetor remains in use in developing countries where vehicle emissions are unregulated and diagnostic and repair infrastructure is sparse. Fuel injection is gradually replacing carburetors in these nations too as they adopt emission regulations conceptually similar to those in force in Europe, Japan, Australia, and North America.

Many motorcycles still utilize carburetored engines, though all current high-performance designs have switched to EFI.

NASCAR finally replaced carburetors with fuel-injection, starting at the beginning of the 2012 NASCAR Sprint Cup Series season.

Electric pump

In many modern cars the fuel pump is usually electric and located inside the fuel tank fuel tank. The pump creates positive pressure in the fuel lines, pushing the gasoline to the engine. The higher gasoline pressure raises the boiling point. Placing the pump in the tank puts the component least likely to handle gasoline vapor well (the pump itself) farthest from the engine, submersed in cool liquid. Another benefit to placing the pump inside the tank is that it is less likely to start a fire. Though electrical components (such as a fuel pump) can spark and ignite fuel vapors, liquid fuel will not explode (seeflammability limit) and therefore submerging the pump in the tank is one of the safest places to put it. In most cars, the fuel pump delivers a constant flow of gasoline to the engine; fuel not used is returned to the tank. This further reduces the chance of the fuel boiling, since it is never kept close to the hot engine for too long.

The ignition switch does not carry the power to the fuel pump; instead, it activates a relay which will handle the higher current load. It is common for the fuel pump relay to become oxidized and cease functioning; this is much more common than the actual fuel pump failing. Modern engines utilize solid-state control which allows the fuel pressure to be controlled via pulse-width modulation of the pump voltage. This increases the life of the pump, allows a smaller and lighter device to be used, and reduces electrical load.

Cars with electronic fuel injection have an electronic control unit (ECU) and this may be programmed with safety logic that will shut the electric fuel pump off, even if the engine is running. In the event of a collision this will prevent fuel leaking from any ruptured fuel line. Additionally, cars may have aninertia switch (usually located underneath the front passenger seat) that is "tripped" in the event of an impact, or a roll-over valve that will shut off the fuel pump in case the car rolls over.

Some ECUs may also be programmed to shut off the fuel pump if they detect low or zero oil pressure, for instance if the engine has suffered a terminal failure (with the subsequent risk of fire in the engine compartment).

The fuel sending unit assembly may be a combination of the electric fuel pump, the filter, the strainer, and the electronic device used to measure the amount of fuel in the tank via a float attached to a sensor which sends data to the dash-mounted fuel gauge. The fuel pump by itself is a relatively inexpensive part. But a mechanic at a garage might have a preference to install the entire unit assembly.

Six-stroke engine

The six-stroke engine is a type of internal combustion engine based on the four-stroke engine, but with additional complexity intended to make it more efficient and reduce emissions. Three types of six-stroke engine have been developed since the 1890s:

In the first approach, the engine captures the heat lost from the four-stroke Otto cycle or Diesel cycle and uses it to power an additional power and exhaust stroke of the piston in the same cylinder. Designs use either steam or air as the working fluid for the additional power stroke. The pistons in this type of six-stroke engine go up and down three times for each injection of fuel. There are two power strokes: one with fuel, the other with steam or air.

The second approach to the six-stroke engine uses a second opposed piston in each cylinder that moves at half the cyclical rate of the main piston, thus giving six piston movements per cycle. Functionally, the second piston replaces the valve mechanism of a conventional engine but also increases the compression ratio.

Internal combustion engine

The internal combustion engine (ICE) is an engine in which the combustion of a fuel (normally a fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine the expansion of the high-temperature and high-pressure gases produced by combustion apply direct force to some component of the engine. The force is applied typically to pistons, turbine blades, or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy. The first commercially successful internal combustion engine was created by Étienne Lenoir.

The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the six-stroke piston engine and the Wankel rotary engine. A second class of internal combustion engines use continuous combustion: gas turbines, jet engine sand most rocket engines, each of which are internal combustion engines on the same principle as previously described.

The ICE is quite different from external combustion engines, such as steam or Stirling engines, in which the energy is delivered to a working fluid not consisting of, mixed with, or contaminated by combustion products. Working fluids can be air, hot water, pressurized water or even liquid sodium, heated in a boiler. ICEs are usually powered by energy-dense fuels such as gasoline or diesel, liquids derived from fossil fuels. While there are many stationary applications, most ICEs are used in mobile applications and are the dominant power supply for cars, aircraft, and boats.