Reference
Coolant & automotive fluid glossary
35 terms explained without jargon. Written for buyers, workshops and anyone who has to read a coolant specification sheet and make a decision.
A
- AdBlueTrade name for diesel exhaust fluid
- A registered trade name for the urea solution used in selective catalytic reduction systems on diesel vehicles. Chemically it is a 32.5 percent aqueous urea solution meeting ISO 22241. Sold as DEF in North America and AUS 32 in parts of Asia. Read the full guide →
- AntifreezeConcentrated glycol product
- A concentrated glycol-based product intended to be mixed with water before use. It lowers the freeze point and raises the boil point of the resulting coolant, and carries the corrosion inhibitor package.
- Ash contentResidue left after ignition, percent by mass
- A measure of non-combustible residue in coolant. High ash promotes deposit formation in the radiator and on heat transfer surfaces. Coolant specifications typically limit ash to 2.0 percent or less.
- ASTM D1384Glassware corrosion test
- A standard test that exposes six metals — copper, brass, solder, steel, cast iron and cast aluminium — to coolant under heat for 336 hours and measures weight loss per coupon. It is the primary evidence that a coolant protects a mixed-metal cooling system. Read the full guide →
- ASTM D1881Foaming tendency test
- A test measuring how much foam a coolant generates and how quickly it breaks. Excessive foaming reduces heat transfer and can contribute to cavitation damage at the water pump.
- ASTM D4340Cast aluminium heat-rejection corrosion test
- A test that measures corrosion of cast aluminium under heat flux, simulating conditions in an aluminium cylinder head. Results are expressed in mg/cm² and are typically required to be at or below 1.0.
- AUS 32Asian designation for DEF
- The designation used in several Asian markets for a 32.5 percent aqueous urea solution for SCR systems. Functionally identical to DEF and AdBlue when it meets ISO 22241. Read the full guide →
B
- Boil-overCoolant escaping as the system boils
- The condition where coolant temperature exceeds the boiling point at system pressure and the fluid erupts through the pressure cap into the overflow bottle. The dominant cooling failure mode in tropical and subtropical markets. Read the full guide →
- BorateInorganic inhibitor and pH buffer
- A boron-based salt used as a corrosion inhibitor and, importantly, as a pH buffer. Borate keeps coolant alkaline for longer, resisting the acidification that consumes inhibitor packages. Restricted in a few markets, highly effective in most.
C
- CarboxylateOrganic acid used in OAT chemistry
- The family of organic acids used as inhibitors in OAT coolants, including 2-ethylhexanoic acid, sebacic acid and benzoic acid. They deplete slowly, which is why OAT products have long service lives.
- CavitationVapour bubble collapse eroding metal
- The formation and violent collapse of vapour bubbles on a vibrating metal surface, most often on wet cylinder liners. It erodes metal and can perforate a liner. Heavy-duty coolants include specific additives to prevent it.
- ChlorideCorrosive ion, mg/kg
- Chloride ions accelerate pitting corrosion of aluminium. They enter through hard or contaminated water and through raw materials. Coolant specifications usually limit chloride to 25 mg/kg or less.
- COACertificate of Analysis
- A document issued for a specific production batch, reporting measured values for parameters such as freeze point, boil point, pH, density and reserve alkalinity. It is batch-specific evidence, unlike a generic specification sheet. Read the full guide →
- CoolantThe fluid in the cooling system
- The ready-to-use fluid circulating through an engine cooling system, comprising water, glycol for freeze and boil point control, and the corrosion inhibitor package. Distinct from antifreeze, which is the concentrate before mixing.
D
- DEFDiesel exhaust fluid
- A 32.5 percent aqueous urea solution injected into the exhaust of diesel vehicles to reduce nitrogen oxides through selective catalytic reduction. Regulated by ISO 22241. Read the full guide →
- DepletionGradual consumption of inhibitors
- The process by which corrosion inhibitors are consumed as they protect metal surfaces. Once depleted, the coolant still looks normal but no longer prevents corrosion. Depletion rate is the main determinant of service life.
- DOT3 / DOT4Brake fluid performance grades
- Performance classifications for hydraulic brake fluid. DOT4 has a higher minimum boiling point than DOT3, giving a greater safety margin under heavy braking. Both are glycol-ether based and are hygroscopic, meaning they absorb water over time.
E
- Ethylene glycolThe base of most coolant
- The most common base chemical for engine coolant. It lowers the freeze point and raises the boil point of the water it is mixed with. Toxic if ingested, so it must be handled and disposed of carefully.
F
- Freeze pointTemperature at which coolant begins to solidify
- The lowest temperature at which the coolant remains liquid. It improves as glycol concentration rises, up to about 70 percent by volume, and then worsens, because neat glycol freezes near minus 13 degrees Celsius. Read the full guide →
G
- GHSGlobally Harmonized System for chemical labelling
- The international system for classifying and labelling chemicals, defining the sixteen-section safety data sheet format and the hazard pictograms used on labels. SDS documents are written to GHS. Read the full guide →
H
- Hard waterWater with high mineral content
- Water containing dissolved calcium and magnesium salts. Using it to dilute coolant deposits scale on hot metal surfaces, which insulates them and reduces radiator efficiency. Deionised or distilled water should be used instead.
- HOATHybrid organic acid technology
- A coolant chemistry combining OAT carboxylates with a small amount of silicate and sometimes nitrite, aiming to combine OAT's long life with the fast initial protection of inorganic inhibitors. Read the full guide →
I
- IATInorganic additive technology
- The traditional coolant chemistry using silicates, phosphates, borates and nitrites. It protects quickly but depletes in roughly two years, and silicate can gel and drop out of solution. Read the full guide →
- ISO 22241The diesel exhaust fluid standard
- The international standard defining the quality requirements for diesel exhaust fluid, including urea concentration, refractive index, alkalinity and limits on metal and biuret content. Read the full guide →
- ISO 9001Quality management system standard
- An internationally recognised standard for quality management systems. A factory holding ISO 9001 has documented and audited processes covering design, production, inspection and corrective action.
O
- OATOrganic acid technology
- Coolant chemistry based on organic acid carboxylates rather than inorganic salts. It depletes very slowly and supports five-year or longer service intervals, but builds its protective film more slowly than IAT or Si-OAT. Read the full guide →
P
- pHAcidity or alkalinity
- A measure of acidity on a scale from 0 to 14. Fresh coolant is alkaline, typically between 7.5 and 11.0. As inhibitors deplete the pH falls, and a falling pH is the clearest early sign that a coolant is nearing the end of its life.
- Premixed coolantReady-to-use coolant
- Coolant supplied already diluted to the correct ratio with deionised water, so it can be poured in directly. It removes the risk of an incorrect mix ratio and the risk of introducing hard water into the system.
R
- Reserve alkalinityCapacity to neutralise acid, in millilitres
- A measure of how much acid the coolant can absorb before its pH drops significantly. It indicates how much service life remains in the inhibitor package. Specifications usually require at least 5 mL, with 10 mL or more preferred.
S
- SCRSelective catalytic reduction
- The exhaust after-treatment system used on Euro V, Euro VI and equivalent diesel engines. It injects diesel exhaust fluid into the exhaust stream, where it reacts to convert nitrogen oxides into nitrogen and water. Read the full guide →
- SDSSafety data sheet
- A sixteen-section document describing a chemical product's hazards, composition, safe handling, first aid, storage and transport classification, written to the GHS format. Formerly known as MSDS. Read the full guide →
- Si-OATSilicate plus organic acid technology
- A coolant chemistry pairing silicate for immediate aluminium protection with OAT carboxylates for long life. Common in European specifications such as VW TL 774 and MB 325.0. Read the full guide →
- Snap testField check of coolant quality
- An informal inspection covering appearance, smell and texture. Milky or oily appearance points to contamination, a sweet burnt smell suggests overheating, and gritty sediment suggests mixing incompatible products or hose degradation.
T
- ThermostatEngine temperature regulator
- A valve that controls coolant flow to the radiator to hold the engine at its design temperature. A thermostat stuck closed causes rapid overheating; stuck open causes slow warm-up and poor heater performance.
W
- Wet linerCylinder liner in direct contact with coolant
- A cylinder liner whose outer surface is directly exposed to coolant. Wet liners in heavy-duty diesel engines are vulnerable to cavitation erosion, which is why heavy-duty coolants include dedicated cavitation protection.