Coolant

Coolant is a liquid circulated through the engine and radiator to keep the engine cool. Water is excellent for extracting heat from the engine, but on its own it has a low boiling point, can freeze in cold weather, and causes rapid corrosion of most metals. Therefore it needs an additive to raise the boiling point, lower the freezing point, and prevent corrosion. The additive is generically called coolant, or sometimes just antifreeze. The chemicals used to provide the antifreeze properties are completely independent of those that inhibit corrosion.

Antifreeze

Over 90% of concentrated coolant is a polyol, a chemical like an alcohol but with multiple hydroxyl groups instead of the one that alcohols have, and is used to raise the boiling point and lower the freezing point when mixed in to water. Almost all coolants use one or more of three chemicals to provide the antifreeze effect:

Ethylene glycol: most modern coolants have only ethylene glycol. It is acutely toxic and sweet, which means it is dangerous for children, pets, and wildlife if spilled in the open, but it is cheap and effective.
Propylene glycol: started to become popular as a less toxic alternative to ethylene glycol, but now uncommon. Modern coolants usually use bittering agents to make ethylene glycol unpalatable.
Glycerol: sometimes mixed with ethylene glycol, it is a less toxic and more environmentally friendly antifreeze.

Pure ethylene glycol has a freezing point of -12C, which is not low enough for reliable use in the UK. When mixed with water, the freezing point becomes much lower, below -50C at certain concentrations. Most coolants are formulated to be mixed with approximately 50% water for protection to around -37C. Propylene glycol has a lower freezing point. Glycerol is a viscous liquid at room temperature and freeze at 18C, so it must be used in mixtures with other antifreezes and water.

On the whole there is nothing much to worry about. Coolants all have much the same antifreeze regardless of anything else. Choose one with glycerol if you really want to be eco, although it will still be mostly ethylene glycol.

Corrosion inhibitors

There are many different chemicals used as corrosion inhibitors in engine coolants. These make up less than 10% of the coolant, with over 90% being the antifreeze. There are two main classes of corrosion inhibitor, IAT and OAT, but with a bewildering array of combinations, proprietary formulations, names, and colours.

IAT, Inorganic Acid Technology. Not actually acids, these are salts of inorganic acids, usually quite alkaline, such as amines, nitrites, phosphates, and silicates. These are the traditional corrosion inhibitors, effective for all metals, and have an engine service life of up to 2 years or 30,000 miles. Some downsides are sludging and rusting from as they quickly lose effectiveness, and water pump damage from silicate precipitation.
HOAT, Hybrid Organic Acid Technology. A confusing term originally used for coolants with inorganic inhibitors that also contained some organic inhibitors, designed to extend the short service life of pure IAT coolants. The term is now sometimes applied to modern coolants such as the silicated OAT group, but these are a very different set of inhibitors.
OAT, Organic Acid Technology. Again, not acids, but salts of organic acids, such as carboxylates, sebacates, and 2-EHA. These give good protection to ferrous metals and aluminium alloys, but poor for "yellow" metals which use solders. They have a longer service life, typically quoted at 5 years or 100,000 miles. These are not safe for use in older engines which contains brass or copper. Early versions in the US were dogged by reports of corrosion and engine damage, and one of the inhibitors (2-EHA) has been implicated in damage to silicone-based seals and gaskets. Many modern formulations do not use 2-EHA or have very low levels.
Si-OAT, Silicate Organic Acid Technology. "Universal" coolants, safe to be mixed with any other coolant. These are now added to many new European cars.
Other types, such as P-OAT and PSi-OAT. P-OAT coolants are found in many Japanese brands, designed for very long service life but somewhat sensitive to use conditions such as hard water. PSi-OAT is relatively new, but becoming common in European engines.

Mixing

It is generally recommended not to mix different coolants even if they are the same colour, although more modern types are increasingly becoming safe to mix with anything.

The "classic" IAT coolants (eg. G11) should never be mixed with the first-generation "plain" OAT coolants (eg. G12). OAT coolants were then improved (ie. G12+, G30, etc) so that they could be safely mixed with older IAT coolants and the original OAT coolants, albeit potentially with some loss of protection length. These were further developed to Si-OAT and PSi-OAT coolants, which are also generally safe to mix with most older coolants. Mixing coolants of radically different colours may produce an unpleasant-looking brown result.

HOAT, PHOAT, etc., may not be compatible with other types, and there are many coolants that don't fit cleanly into any of these categories. Again the safest advice is never to mix different types.

Although VW G13 and other coolants containing glycerol contain a different combination of antifreezes, it is usually safe to mix them with any other type.

Identification

Colours

Coolants or antifreezes are usually dyed a strong colour. Originally this was an almost-fluorescent green for easy identification. Later formulations with different corrosion inhibitors were then dyed different colours, but a universal system has never been adopted. Colours vary for the same chemical makeup between brands and in different regions of the world, and the same colours may be used for different chemical makeups.

Based on the colour alone, you might be able to take a guess at what the coolant is, but no guarantees:

Orange: almost always equivalent to GM Dex-Cool, 2-EHA-based
Yellow: G05-type lobrid ("heavy duty"), or "universal" (eg. Prestone), or plain OAT (eg. many trucks)
Yellow-green: probably Renault type D, used to be plain OAT, now Si-OAT (some POAT "green" coolants are pale and yellowish)
Red: usually plain OAT, sometimes 2-EHA-based, often more complex carboxylic acids or mixtures, to G12 or G12+ standard (note that OEM Toyota coolant for very old models is red and IAT)
Pink: usually an OAT that is safe to mix with other types of coolant, may be Si-OAT or PSi-OAT, to G12+, G12++, or G12 EVO standard, but Japanese (eg. Toyota) pink coolant is PHOAT
Violet or lilac: usually Si-OAT or PSi-OAT, to G12++ or G12 EVO standard
Purple: glycerin-based Si-OAT to G13 standard
Blue, blue-green: may be IAT or lobrid OAT (common for European and Japanese OEM coolants)
Green: may be IAT or P(H)OAT (usually Japanese), or Si-OAT (BMW/Toyota HT-12)

Brands and codes

Coolant colour is not a reliable guide to what is actually in the coolant, so it is important both to check what is already in the system and what a new product is. Mixing certain coolant types can produce sludge or thickening, or simply reduce the effectiveness and lifetime. Different manufacturers and car brands use their own or semi-standard codes, but mostly these are a few basic types:

IAT: VW G11 TL 774C (blue), Comma Super Coldmaster, previously Classic (blue), Halfords Silicate (= Comma Super Coldmaster), ECP Triple QX Blue (blue), Granville Subzero (blue), Bluecol 2yr (blue)
HOAT: Comma/Glysantin G05 (yellow, with nitrites) BMW LC-87 (blue), Comma/Glysantin G48 (blue/green), Halfords NAP-free (= Comma G48), Citroen/Peugeot/Opel/G33 (blue)
- PHOAT: Honda Type 2 (blue), Renault Type E (blue)
OAT, VW G12 TL 774D (red), VW G12+ TL 774F (pink), Comma Super Longlife Red (red), Halfords OAT (red), Comma/Glysantin G30 (violet), Glysantin G33 (blue/green), Glysantin G34 (orange), ECP Triple QX Red (red), ECP Triple QX Orange (orange), Havoline XLC (orange), Granville Zerocol (red), Bluecol 5yr (red), old Renault Type D (yellow-green)
Si-OAT: VW G12++ TL 774G (pink), VW G13 TL 774J (purple, with glycerin), ECP Triple QX Purple (purple, with glycerin), Havoline QFC (red, with glycerin), BMW LC-18 (green), Glysantin G40 (violet), new Renault Type D (yellow-green)
PSi-OAT: VW G12EVO (pink), Glysantin G64 (green), Glysantin G65 (pink)

Lotus

Elise S1

The Elise S1 contains a Rover K-series cast aluminium engine. The Elise S1 owner's handbook recommends only using Unipart Superplus, which was green, and a change interval of two years. Unipart Superplus is no longer available, but similar non-OAT coolants are. The "official" Xpart coolant for the K-series engine is now a red OAT coolant, available at EliseParts. Many S1's have been switched to the red OAT type at some stage of their lives, which should last approximately 5 years before needing to be changed.

Either IAT or OAT coolants can be used in the Elise S1. The important thing is not to mix them, and to flush the system if changing from one to the other. Modern OAT coolants ("pink", G12+/++, G30, etc) are more forgiving about being mixed with other types. A HOAT coolant (Unipart AFC) was briefly listed as the official coolant for the K-series in the MGF around 2000. Check the levels regularly and refresh at the recommended interval.

Elise S2

Rover engined cars use the same coolant as the S1.

Evora

The recommended coolant for the Evora is red PETRONAS Paraflu Up (previously orange Havoline XLC), an OAT coolant. The change interval is four years. USA?

Coolant

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