When it comes to cleaning the engine bay—that is, the external surfaces of the engine and surrounding components—“engine degreaser” products come in several distinct types, each suited to a different job/condition. Below you’ll find a breakdown of the major types, what to use when, their pros/cons, and then some good product examples you can pick from.

✅ Types of External Engine‐Degreasers

Here are the main categories of external engine degreasers:

1. Solvent-based / heavy-duty degreasers

What they are: These use strong chemical solvents (e.g., mineral spirits, aromatic hydrocarbons, glycol esters) to dissolve thick, baked-on grease, oil, sludge and grime. 

When to use: For neglected or very dirty engine bays (oil leaks, heavy buildup, old grime), especially metal parts.
Pros: Very strong cleaning power; works fast and on heavy deposits. 
Cons/Risks: Often strong smell, may be flammable, may be harsher on plastics/rubber or painted surfaces; requires good ventilation and care.
Tips: Cover or mask sensitive components (electrical connectors, sensors, exposed wiring). Use when engine bay is cool.
Example formulation note: “Solvent-based engine degreasers contain … hydrocarbon solvents such as glycol esters, mineral spirits and aromatic agents.

2. Water-based / surfactant degreasers

What they are: These use water + surfactants/emulsifiers + alkaline agents to break down grease. They tend to be more “gentle” and environmentally friendlier. 
When to use: Regular maintenance cleaning, newer engines with moderate grime, or when you’re cautious about plastics/rubber and environmental impact.
Pros: Safer for most engine bay materials, less odor, less risk of damage to non-metal parts. 
Cons: Might not be strong enough for heavy oil/grease buildup. May require more scrubbing, dwell time.
Tip: Good for light or moderate grime; for heavy oil leaks or baked sludge you may need the more intense solvent-type.

3. Foam, gel or aerosol / speciality format degreasers

What they are: Variants designed for particular application formats: foam sprays that cling to vertical surfaces, gels for thick buildup, aerosol convenience. 
When to use: If you want targeted cleaning (e.g., parts of the engine bay), for detailing, or when access is tricky and you want something “stickier”.
Pros: Fine for spot cleaning, easier to apply and control; foam can cling. 
Cons: Might not penetrate heavy layers; more expensive per volume; body-wide coverage may be slower.
Tip: Use foam/gel for vertical/hard-to-reach surfaces, for visual tidying, or as follow-up after heavier cleaning.

4. Bio-based / citrus & eco-friendly degreasers

What they are: Degreasers derived from natural sources (citrus terpenes, plant oils) with less harsh chemistry; often marketed as “green”.
When to use: For light to moderate cleaning, in environments where strong solvents are undesirable, or for regular maintenance.
Pros: Lower toxicity, less odor, safer on many surfaces.
Cons: Less aggressive for heavy oil/grease buildup; may cost more.
Tip: Good as a regular upkeep product; for deep cleaning you might need a stronger one.

🔍 How to decide which type you need

Here are some decision cues: