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32-514: It was preceded by Honda XRV650 Africa Twin, which was a lighter, higher specification version made in 1988 and 1989 by Honda Racing Corporation with a 650 cc engine producing 50 hp (37 kW). The much earlier Honda XLV750R was a shaft driven motorcycle. Built in homage to the giant desert racers of the Paris-Dakar Rally, the Africa Twin is a large, dual sport bike, powered by

64-479: A parallel twin engine with a 270° crankshaft (as pioneered by the Yamaha TRX850 ). The new engine was cheaper to build, lighter and more compact, allowing both a shorter wheelbase and easier placement of inlet and exhaust manifolds. Also, the 270° engine's "big bang" concept allows excellent rear wheel grip to the track surface. Honda XRV650 The XRV650 Africa Twin is an enduro motorcycle produced by

96-428: A bore of 79 mm, the pistons have a stroke of 66 mm and a compression ratio of 9.4:1. Each cylinder head has a chain-driven camshaft that controls one exhaust and two intake valves. The engine and transmission block unit is vertically separated and made of aluminum alloymanufactured. The crankshaft transmits its torque via gears to a multi-plate wet clutch on the input shaft of the gearbox . The gearbox has five gears and

128-562: A chain guide and crash bars for the radiators. However, the standard air filter box proved to be problematic. In the second year of the RD 03 Marathon, this was converted to an air intake behind the tank cap. Compression ratio The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values. A fundamental specification for such engines, it can be measured in two different ways. The simpler way

160-474: A compression ratio of 6.5 or lower. The petrol-paraffin engine version of the Ferguson TE20 tractor had a compression ratio of 4.5:1 for operation on tractor vaporising oil with an octane rating between 55 and 70. Motorsport engines often run on high-octane petrol and can therefore use higher compression ratios. For example, motorcycle racing engines can use compression ratios as high as 14.7:1, and it

192-480: A cooling fan as additional cooling for coolant temperatures above 100 °C. The motorcycle was successfully used as the RD 03 Marathon in 1989 and 1990 by Honda France in the marathon class of the Paris-Dakar Rally . A rear tank and an enlarged main tank were installed for the long stages. Many other changes were made to this model such as an engine guard with a water compartment, a modified swingarm for attaching

224-476: A deflection system and is infinitely adjustable in terms of both spring preload and damping. The engine is cooled with a mixture of water and antifreeze, which dissipates the combustion heat from the two cylinders to the ambient air via two radiators. A coolant pump driven by the oil pump conveys the hot coolant to the coolers and back via a thermostat valve , with the thermostat only fully opening from an operating temperature of 95 °C. The right-hand radiator has

256-414: A given mass of air–fuel mixture due to its higher thermal efficiency . This occurs because internal combustion engines are heat engines , and higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature. In petrol (gasoline) engines used in passenger cars for

288-460: A softly tuned V-twin engine . It has twin headlights, a windscreen, and a long dual seat which stretches back from the tank to an aluminium grabrail and plastic coated luggage rack. An aluminium bashplate protects the bottom of the engine from flying rocks and impacts. The engine is a 742 cc, liquid-cooled V-twin with a single overhead camshaft , 6-valves (3 per cylinder) and four spark plugs (2 per cylinder). The long-travel suspension insulates

320-536: A two-piston caliper and at the rear with a disc brake with a single-piston caliper. The motorcycle was only available in the Honda Racing Corporation (HRC) color combination (white-red-blue) until 1991. The water-cooled four-stroke engine is mounted transversely in the frame so that the two cylinders of the V-engine are positioned one behind the other at an angle of 52° to the crankshaft. The two cylinders have

352-405: A weight of 220 kg with a full tank, the travel enduro was easy to maneuver both on and off-road. In Germany, because of the cheaper type class, the power was reduced by throttling from 57 hp to 50 hp. The top speed is 165 km/h at a speed of 7,640 rpm . The aluminum rims have steel spokes and are fitted with cross-ply tires at the front and rear. The RD 03 decelerates at the front via a disc brake with

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384-464: Is a polytropic value for the ratio of specific heats for the combustion gases at the temperatures present (this compensates for the temperature rise caused by compression, as well as heat lost to the cylinder) Under ideal (adiabatic) conditions, the ratio of specific heats would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used. For example, if

416-415: Is common to find motorcycles with compression ratios above 12.0:1 designed for 95 or higher octane fuel. Ethanol and methanol can take significantly higher compression ratios than gasoline. Racing engines burning methanol and ethanol fuel often have a compression ratio of 14:1 to 16:1. In a piston engine , the static compression ratio ( C R {\displaystyle \mathrm {CR} } )

448-426: Is connected to the rear wheel via a chain. The chassis consists of a single-loop tubular steel frame with a double upper section made of rectangular profiles and a classic rear frame. The two stanchions of the telescopic fork have a diameter of Ø 43 mm and a spring travel of 230 mm. They are connected to each other via a fork bridge above the front wheel. The aluminum shock absorber on the rear wheel swing arm has

480-447: Is done to increase fuel efficiency while under varying loads. Variable compression engines allow the volume above the piston at top dead centre to be changed. Higher loads require lower ratios to increase power, while lower loads need higher ratios to increase efficiency, i.e. to lower fuel consumption. For automotive use this needs to be done as the engine is running in response to the load and driving demands. The 2019 Infiniti QX50

512-457: Is the static compression ratio : the ratio of the volume of the cylinder when the piston is at the bottom of its stroke to that volume when the piston is at the top of its stroke . The dynamic compression ratio is a more advanced calculation which also takes into account gases entering and exiting the cylinder during the compression phase. A high compression ratio is desirable because it allows an engine to extract more mechanical energy from

544-409: Is the first commercially available car that uses a variable compression ratio engine. The static compression ratio discussed above — calculated solely based on the cylinder and combustion chamber volumes — does not take into account any gases entering or exiting the cylinder during the compression phase. In most automotive engines, the intake valve closure (which seals the cylinder) takes place during

576-518: Is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke , and the volume of the combustion chamber when the piston is at the top of its stroke . It is therefore calculated by the formula C R = V d + V c V c {\displaystyle \mathrm {CR} ={\frac {V_{d}+V_{c}}{V_{c}}}} where V d {\displaystyle V_{d}} can be estimated by

608-538: Is used. This can reduce efficiency or damage the engine if knock sensors are not present to modify the ignition timing. Diesel engines use higher compression ratios than petrol engines, because the lack of a spark plug means that the compression ratio must increase the temperature of the air in the cylinder sufficiently to ignite the diesel using compression ignition . Compression ratios are often between 14:1 and 23:1 for direct injection diesel engines, and between 18:1 and 23:1 for indirect injection diesel engines. At

640-760: The Japanese manufacturer Honda from 1988 to 1989. The enduro is derived from the design of the Honda XL600V Transalp and is powered by the V-twin cylinder engine of the Honda NT650 Hawk. In response to the motorsport successes of the BMW R80 GS in the early 1980s, the Honda Racing Corporation (HRC) was commissioned in 1984 to develop a desert-capable, off-road motorcycle for the Paris-Dakar Rally. In 1985

672-401: The cylinder volume formula: V d = π 4 b 2 s {\displaystyle V_{d}={\tfrac {\pi }{4}}b^{2}s} where Because of the complex shape of V c {\displaystyle V_{c}} it is usually measured directly. This is often done by filling the cylinder with liquid and then measuring the volume of

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704-553: The NXR750V was presented with a water-cooled twin-cylinder with a displacement of 779 cm³ and a rated output of 48 kW (65 hp). This motorcycle won the Paris-Dakar Rally four times in a row from 1986 to 1989. The Africa Twin XRV 650 with manufacturer code RD 03 is not based on the works rally motorcycle NXR 750, but has adopted its look after the success of the NXR in the 1987 Paris-Dakar Rally. Despite

736-545: The XRV gained an improved seat and clutch, larger silencer, modified upper fairing and luggage rack. However, the rear shock absorber lost some of its adjustability. In 2000 the Honda XRV750 Africa Twin ceased production. XRVs still in the showrooms were sold and registered until 2003 but there is no XRV with a VIN that is newer than 2000. Nowadays good second hand examples are very much sought after among aficionados. Several aftermarket products exist with which to equip

768-436: The bike such as crash bars to protect the vehicle's plastics and tank from damage in a low speed fall. The later XRV's instruments feature a large trip computer LCD display mounted above the conventional speedometer and tachometer, styled like Dakar racers' navigational displays, and incorporates a range of extra electronic timers and trip meters. Africa Twins were originally built with a V-twin layout, but current models use

800-561: The compression phase (i.e. after bottom dead centre , BDC), which can cause some of the gases to be pushed back out through the intake valve. On the other hand, intake port tuning and scavenging can cause a greater amount of gas to be trapped in the cylinder than the static volume would suggest. The dynamic compression ratio accounts for these factors. The dynamic compression ratio is higher with more conservative intake camshaft timing (i.e. soon after BDC), and lower with more radical intake camshaft timing (i.e. later after BDC). Regardless,

832-549: The cylinders. Engines using port fuel-injection typically run lower boost pressures and/or compression ratios than direct injected engines because port fuel injection causes the air–fuel mixture to be heated together, leading to detonation. Conversely, directly injected engines can run higher boost because heated air will not detonate without a fuel being present. Higher compression ratios can make gasoline (petrol) engines subject to engine knocking (also known as "detonation", "pre-ignition", or "pinging") if lower octane-rated fuel

864-459: The dynamic compression ratio is always lower than the static compression ratio. Absolute cylinder pressure is used to calculate the dynamic compression ratio, using the following formula: P cylinder = P atmospheric × CR γ {\displaystyle P_{\text{cylinder}}=P_{\text{atmospheric}}\times {\text{CR}}^{\gamma }} where γ {\displaystyle \gamma }

896-465: The lower end of 14:1, NOx emissions are reduced at a cost of more difficult cold-start. Mazda's Skyactiv-D , the first such commercial engine from 2013, used adaptive fuel injectors among other techniques to ease cold start. The compression ratio may be higher in engines running exclusively on liquefied petroleum gas (LPG or "propane autogas") or compressed natural gas , due to the higher octane rating of these fuels. Kerosene engines typically use

928-403: The past 20 years, compression ratios have typically been between 8:1 and 12:1. Several production engines have used higher compression ratios, including: When forced induction (e.g. a turbocharger or supercharger ) is used, the compression ratio is often lower than naturally aspirated engines . This is due to the turbocharger or supercharger already having compressed the air before it enters

960-509: The rider from uneven surfaces. The brakes are twin discs at the front and single disc at the rear. In December 1989 the original Honda XRV750 Africa Twin was launched, which became known as the 1990 model. In 1990 was updated. In 1992 the Tripmaster computer was added. In 1993 the motorcycle had a major redesign including new frame, body work plastics, fuel tank, engine modifications and a lower seat. Nevertheless, it gained weight slightly. In 1996

992-424: The static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be 7.5 × atmospheric pressure, or 13.7  bar (relative to atmospheric pressure). The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have

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1024-417: The used liquid. Most engines use a fixed compression ratio, however a variable compression ratio engine is able to adjust the compression ratio while the engine is in operation. The first production engine with a variable compression ratio was introduced in 2019. Variable compression ratio is a technology to adjust the compression ratio of an internal combustion engine while the engine is in operation. This

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