What Is a Starting Engine?

A starting engine—often called a “pony engine”—is a small auxiliary engine used to crank and help bring a much larger main engine to life, typically by turning it over and warming its oil and coolant. In modern cars this role is handled by an electric starter motor, but starting engines were common on large diesel equipment and still appear in specialized or legacy machinery. Understanding the term matters because it’s frequently confused with other starting systems, from electric starters to aircraft APUs.

Definition and Core Function

A starting engine is a self-contained power unit, usually a small gasoline engine, mechanically linked to a larger prime mover. Its job is to spin the main engine fast enough for combustion to take over, often while preheating fluids to reduce friction and improve cold-weather reliability. Historically, this arrangement allowed heavy-duty diesels—found in tractors, bulldozers, and stationary generators—to start in conditions where batteries or electric starters were inadequate.

Where You’ll Encounter Starting Engines

Starting engines aren’t common in everyday passenger vehicles, but they have been used widely in heavy machinery and certain specialized contexts. Here are typical settings where the term has applied.

• Heavy equipment and agriculture: Older Caterpillar tractors and bulldozers (e.g., D2, D4, D6) used gasoline “pony” engines to crank and warm large diesel engines.
• Stationary and remote-power diesel sets: Some legacy generator installations employed a small starting engine for dependable off-grid starts.
• Aviation (historical/analogous systems): While not usually called “starting engines,” early and military aircraft used small turbine or cartridge starters; modern jets rely on air turbine starters powered by an APU or ground cart.
• Cold climates and field operations: Where batteries struggle, a mechanically simple auxiliary engine could provide more reliable cranking and preheat.

Today, most on-road vehicles use electric starters, and many heavy diesels rely on high-capacity batteries, intake heaters, glow plugs, or pneumatic/hydraulic starters—reducing the need for dedicated starting engines.

How a Starting Engine Works

Although designs vary, the operating sequence for a traditional pony engine is relatively standard and emphasizes warming and mechanical engagement.

1. Preparation: Check fuels (often gasoline for the pony, diesel for the main), oil levels, and coolant. Ensure the main engine is unloaded and controls are set for start.
2. Pony engine start: Use magneto ignition/choke to start the small engine; let it stabilize.
3. Warm-up: Circulate coolant and oil, sometimes via integrated pumps, to raise temperatures and reduce viscous drag in the main engine.
4. Engagement: Use a clutch or reduction gear to connect the pony engine to the main engine’s flywheel or ring gear, spinning it to starting RPM.
5. Main engine ignition: Once the main engine fires and runs steadily, disengage the pony engine and shut it down.

This process minimizes electrical demand, improves cold-start reliability, and reduces wear by ensuring the main engine isn’t cranked “dry” or at low temperatures.

Types and Related Systems

The phrase “starting engine” is specific, but it sits within a broader family of engine-start technologies. Understanding the differences prevents confusion.

• Gasoline pony engine (true starting engine): A small, separate engine that mechanically cranks and warms a larger diesel engine.
• Electric starter motor (most cars/trucks): An electric motor engages the flywheel to crank the engine; not a “starting engine,” though it serves the same purpose.
• Air-start systems (marine/large diesels): Compressed air spins the engine directly; common on big marine engines and some industrial diesels.
• Hydraulic starters: Hydraulic pressure cranks the engine; used where sparks or batteries are undesirable.
• Air turbine starters and APUs (aviation): An APU (a small gas turbine) provides pneumatic/electrical power to spin main engines via air turbine starters; functionally similar to a starting engine but generally not labeled as such.
• Cartridge starters (historical aviation): A combustible charge drives a small turbine or mechanism to spin the engine—rugged and independent of electrical systems.

While these systems differ in power source and mechanism, they all aim to achieve reliable, repeatable cranking under varied conditions.

Advantages and Drawbacks

Starting engines offered practical benefits when electrical systems were limited, but they also introduced complexity.

• Advantages: Strong cold-start performance; reduced battery dependence; preheats oil/coolant; robust in field conditions with minimal electrics.
• Drawbacks: Added weight and maintenance; mixed fuels (gasoline plus diesel); more components to fail; slower and less convenient than modern electric or pneumatic starts.

These trade-offs explain why pony engines declined as batteries, starters, and engine preheat technologies improved.

Maintenance and Safety Notes

Proper care extends the life of both the starting engine and the main engine and reduces hazards during engagement.

• Fuel and ignition: Keep fresh gasoline for the pony engine; maintain magneto/spark components and carburetion.
• Lubrication and cooling: Verify oil levels and coolant circulation in both engines; warming before engagement reduces wear.
• Engagement checks: Ensure the clutch/gear linkage is adjusted to prevent gear clash or partial engagement.
• Ventilation and fire safety: Operate in well-ventilated areas; avoid starting-fluid misuse; keep extinguishers accessible.
• Cold-weather workflow: Use block heaters if available; allow sufficient pony engine warm-up for effective heat transfer.

Attention to these basics improves start success rates and minimizes mechanical stress on aging equipment.

Common Misconceptions

Terminology often causes confusion, especially across automotive, industrial, and aviation contexts.

• “Starting engine” vs. “starter motor”: A starting engine is a separate engine; a starter motor is an electric device. They are not interchangeable terms.
• APU equals starting engine: An APU assists starting but is a self-contained turbine generator, not typically called a starting engine.
• Air-start equals starting engine: Air-start systems perform the same function but use compressed air, not a separate engine.

Clarifying these distinctions helps when sourcing parts, manuals, or procedures for legacy machinery or aircraft.

History and Current Status

Pony engines became popular in the mid-20th century for heavy equipment and large diesels, notably on Caterpillar machines. As electrical systems, battery technology, and engine preheat methods advanced, manufacturers shifted to direct electric starting, pneumatic systems, or hydraulic starters. In aviation, cartridge and inertia starters gave way to air turbine starters powered by APUs or ground air carts. Today, true starting engines are largely historical or niche, though they remain in service on preserved equipment and in some remote or legacy installations.

Summary

A starting engine is a small auxiliary engine used to crank and often warm a larger engine, enhancing reliability—especially in cold or off-grid conditions. While largely replaced by electric, pneumatic, or hydraulic starters and by APUs in aviation, the concept remains important historically and in specialized applications where robustness and independence from large electrical systems are priorities.

What does a starter engine do?

The starter motor is responsible for turning the engine over during ignition and allowing everything else to happen. When you turn the ignition on, the starter motor engages and turns the engine over allowing it to suck in air.

How to tell if a starter motor needs replacing?

Common Signs of a Bad Starter

1. The engine won’t turn over. The most common signal that your starter has a problem is if nothing happens when you turn the key or push to start.
2. Unusual noises, such as clinking, grinding and whirring.
3. Intermittent problems starting the car.
4. Smoke coming from under the hood.

What does starting the engine do?

The starter motor spins the flex plate/flywheel which is connected to the crankshaft. Modern cars when you push the start button or turn the key will additionally activate the fuel and ignition systems as the engine spins over to get it running.

How much does it cost to replace a starting motor?

Replacement costs: Replacing a starter motor tends to range from £250 to £600, including parts and labour. The price depends on the car’s make and model and the type of starter motor.