Haldex Clutch

A limited slip coupling which is available for vehicles with automatic all-wheel drivetrains used to activate a secondary axle whenever it is required. Most vehicles equipped with this feature automatically engage the secondary axle as necessary to maintain operational control and stability.


Refer to “Homogenous Charge Compression Ignition” for more information.

Head Airbags

Refer to “Curtain Airbags” for more information.

Head Room

Amount of headspace available within a vehicle’s cab. The measurement is determined from the distance between an occupant’s hip to the top of the interior vehicle roof. This is an especially important consideration in purchasing a new car for taller drivers.

Head Unit

The primary centralized control unit in charge of a vehicle’s entertainment system. Available features controllable from the head unit will vary depending on what options are present in a vehicle.

Headlight Washers

Washer wipers or nozzles which are equipped on a vehicle’s headlights to keep them clean and free from debris. These devices are frequently available on larger vehicles or off-road vehicles that may frequently become dirty.


Bright lamps on the front of a vehicle which aid in improving visibility for drivers during the night or other times of darkness. Most vehicles have two headlights, but some makes or models may have more.


Speakers worn by passengers individually to listen to music from the vehicle sound system without disturbing anyone else. Headsets or headphone ports may be equipped in various areas of a vehicle depending on the make or model.

Heads-Up Display

Device which provides a digital display projected from a vehicle dashboard directly onto the lower part of the windshield. This display projects a variety of vehicle-related information directly in the line of sight for the driver.

Heated Air Temperature Sensor

Device used to determine the current temperature for air which is being directed into the engine. Computer sensors detect air in various spots in and around the engine to report current temperature readings. Ongoing monitoring of temperatures can prevent overheating or related problems.

Heated Mirrors

Sideview mirrors which are equipped with heating coils built into the internal mechanics. These heating coils keep the mirror glass warmer to prevent fog and to reduce snow, sleet, or ice accumulation and improve visibility.

Heated Windshield

A windshield which also includes embedded heating elements to keep the glass temperature warmer. Warm windshields can reduce fogging and prevent snow or sleet from quickly freezing and accumulating on a windshield while driving.

Heated Windshield Wiper Rests

Heating elements situated below a vehicle’s wipers used to melt snow and ice and prevent wipers from freezing to the windshield glass. This feature is often touted as a luxury item and is only available on specific vehicles.


Heat exchanger which is placed in a vehicle’s ventilation system to provide heat to the driver and occupants. Heat is diffused directly from the engine’s cooling system and channeled into an airstream from a built-in fan. This distributes heat out of the cab vents and can be controlled by occupants.

Heater Hoses

Tubing which is used to transport engine coolant from a vehicle’s cooling system to its heater core. These hoses are critical for ongoing heater operation.


Total distance between the highest point of a vehicle and the level ground.

Highway Fuel Economy

The fuel economy measurement for a vehicle taken while driving under highway conditions of relatively stable speeds without stop-and-go operation. This figure is determined from an EPA driving simulation lab test, and it is usually measured in “Miles Per Gallon” or “MPG.” The EPA test provides an objective assessment of a vehicle’s potential fuel efficiency under these conditions.

Hill Holder

Device used to apply brake pressure whenever a vehicle’s clutch is not actively engaged. This adds an additional layer of braking safety during vehicle operation.

Hinge Type

The door hinge type used in an automobile. These include standard hinges, scissor hinges, suicide-style, and gullwing hinges.

Hip Room

Total distance between the lower door panel on the driver’s side of a vehicle to the same lower side door panel on the passenger side. This measurement is used to help determine the width of a cab space.

Homogenous Charge Compression Ignition

Internal combustion engine where both gasoline and air are compressed and fed into the system to allow for an auto ignition function. The compression feeds the necessary fuel to enable the ignition to properly fire and start a vehicle.


Standard unit of measurement for an engine’s overall power and potential. Originally used as a method to equate steam locomotive power to the equivalent number of draft horses hauling potential, the figure remained as a power standard during the transition into motor vehicles in the 20thcentury. The term is frequently referred to as “HP” and is almost always included in a new vehicle’s specifications.


Shorthand for “Heating, Ventilation and Air Conditioning,” the installed system equipped in a vehicle which provides temperature control in the cab.

Hybrid Degree

The extent of hybrid features and functionality equipped in a vehicle. The levels may include full hybrid, mild hybrid, power-assist hybrid, or a plug-in hybrid. Each degree adds additional hybrid function to an automobile.

Hybrid Drivetrain Layout

The specific drivetrain layout available within a hybrid vehicle. Common layouts include a parallel drivetrain, series drivetrain, or a combined drivetrain.

Hybrid Engine

Powertrain which integrates both an electric engine alongside another power source to operate a vehicle. The majority of hybrids also feature some type of combustion engine such as gasoline or diesel. However, additional options like hydrogen have also become increasingly popular within the hybrid line. A gasoline and electric combination remains the most popular type of hybrid vehicle.

Hybrid Gas Engine Performance

Within a hybrid vehicle, the gasoline engine’s specific ability to do work. Standards of measurement for performance include horsepower and torque. Performance factors may also refer to overall efficiency on the gasoline side of the hybrid operation.

Hybrid Performance

The combined ability of a hybrid vehicle to accomplish work within its driving. Using both electric and combustion potential, this includes factors such as its horsepower, torque, and overall efficiency.

Hybrid Power Source

Source of power used within a hybrid vehicle. The most common types of hybrid power include a gasoline and electric combination, a plug-in electric system, a fuel cell power source, or a pneumatic installation.

Hybrid Vehicle

An automobile which is equipped with a combination of an internal combustion engine as well as an electricity-produced engine. Combustion sources may vary and could include a gasoline, ethanol, or diesel engine. A vehicle typically will automatically switch from electric to combustion depending on the driving circumstances and power level.


Organic compounds which are frequently used as a fuel source within combustion engines. These compounds only consist of hydrogen and carbon atoms. The combustion of hydrocarbons provides the necessary power to effectively operate a vehicle. Examples include gasoline and E85.

Hydrogen Fuel Cell

Device which is used to produce energy by harnessing electrochemical reactions from hydrogen and an oxidant and channeling it into a fuel cell. This process can provide an electrical energy source within a vehicle for either full or partial power during operation.