Modifying Car Engines for Aircraft: Challenges and Benefits

The process of converting car engines to be used in aircraft is a fascinating endeavor that involves a mix of innovation, engineering, and a deep understanding of the differences between automotive and aircraft engines. While it can be done, it is by no means a straightforward task. Let's explore the intricacies and benefits of such modifications.

Introduction to Engine Conversion

Starting with an old, air-cooled VW engine, the modification process is relatively simple. You can mount it backwards, attach a propeller where the clutch/torque converter used to go, and use a slightly smaller propeller than most airplanes. However, for other car engines, a propeller speed reduction unit is typically needed since they generate power at 2-3 times what a propeller can use effectively.

Challenges in Engine Conversion

The real challenge in converting a car engine for use in an airplane lies in the fundamental differences between the two systems. Unlike car engines, which are designed to produce bursts of high power for short periods, aircraft engines are built to operate at a steady 60-90% of their maximum power for extended periods. This means that a car engine's low power output is insufficient for aircraft applications, and its durability is significantly lower, much like a racecar engine that needs frequent rebuilds.

As a result, engines like the 300HP Volkswagen engine that weighs at least 400 pounds fully dressed can effectively be regrouped into a 100HP engine for aircraft purposes. A proper 100HP aircraft engine, on the other hand, weighs around 200 pounds, highlighting the significant weight reduction required for aircraft use.

Standards for Automotive and Aviation Engines

While piston engines remain the backbone of both automotive and aviation systems, the standards and applications differ significantly. The key factor in aviation engines is their power-to-weight ratio, which means they are typically air-cooled or have limited water cooling, and they operate at lower RPMs compared to car engines. Aircraft also operate under changing altitudes and air pressures, demanding advanced ignition and fuel mix management, which is more manageable with contemporary electronic systems.

Moreover, aircraft engines must function under varying environmental conditions, including negative G-forces, which are not common in car engines. Aviation engines also usually use direct ignition instead of timing belts, designed for reliability and performance.

Advantages and Disadvantages

The advantages of car engines in aviation applications are compelling. For instance, the Rotax 914, a light aircraft engine with approximately 115 horsepower, can cost between $42,000 and $50,000. By contrast, a Suzuki G-series engine from an old Volkswagen, with around 90 horsepower, can be acquired for a fraction of the cost. Improvements such as better cylinder groups, pistons, valves, and even a compressor could be incorporated to enhance its performance.

Some car engines, like the air-cooled VW Beetle "boxer" engine, are more suitable for conversion due to their low weight and high reliability. Additionally, the Chevrolet air 6 engine from Corvair, while not as popular, can produce a robust 100-115 horsepower, making it a viable option for aircraft.

Specialized companies such as Limbach Sauer Flugmotorenbau and AeroVee offer professional aircraft engines based on VW flat-four designs at a cost that is much lower than classic aircraft engines but higher than DIY conversions. GM LS motors, while used in some aircraft, are less practical due to their considerable weight, making the empty to maximal takeoff weight ratio less favorable.

Examples of Successful Conversions

Subaru flat four and six "boxer" engines are the most popular choices for modern conversions, with engines like the Subaru EJ25 and the Chevrolet Corvair being notable examples. The Kozachok two-seater gyroplane, for instance, uses a 170HP Subaru EJ25 "boxer" engine, which can be sourced from old cars or recycled for $800.

Engine costs aside, the aircraft themselves also present their own challenges. The Kozachok, powered by an automobile motor, has a significantly lower empty weight but slower cruise speed compared to the Robinson R22 helicopter, which uses a Lycoming O320 engine. The Kozachok, at a fraction of the cost of the aircraft, offers excellent fuel efficiency but is limited in performance.

As aircraft size increases, the cost differences between car and aircraft engines become more pronounced. The Okhotnik-3, using a 210HP Subaru EZ30 engine, is a prime example, being not only cheaper but also more fuel-efficient than the Robinson R44, which requires a more powerful Lycoming 540 engine.

In summary, while the process of converting car engines for aircraft is complex, it offers significant cost and operational benefits, particularly for smaller aircraft and gyroplanes. Despite the challenges, the innovative engineering and sustainable cost-effectiveness of these conversions make them an attractive option for flight enthusiasts and aviation professionals alike.