Comparing Combustion Engine and Brushless Motor Performance Fundamentals

Combustion engines and brushless motors serve the same purpose in the radio controlled scene. The purpose of these two technologies is to provide the main source of mechanical power to drive our boats, planes and cars forward. In this comparison we will be looking at  4 different parameters. These parameters will include output RPM, physical engine / motor size, timing and lastly load. We will look at how many fundamentals from combustion engines that we can apply to brushless motors.

Output RPM Performance Comparison

Combustion Engine Comparison

In this first example I would like to do a comparison between my wifes daily driver and mine. We both drive Toyota’s that have 1.8L engines. However, my wifes Toyota Matrix makes 130hp (@6000 RPM) and my Toyota Corolla makes 170hp. (@7600 RPM) Both engines have the same displacement, (physical size) however there’s a significant power difference. Where does the power difference ultimately come from?

As you probably were able to guess from the heading of this paragraph, the difference is in the output RPM. The Toyota 2ZZ-GE engine makes 170hp as it is able to scream all the way to 8200 RPM. The Toyota 1ZZ-FE engine on the right hand side of the image below, is only able to reach a 6400 RPM redline.

Power is produced when a motor or engine is able to output torque at a specific RPM.  Power = Torque x Rotational Speed.  This tells us that as long as both engines in this comparison are able to produce the same amount of torque, the higher RPM output of the 170hp engine is what ultimately allows it to get there.  A 7600 RPM peak power output is about 27% higher then a 6000RPM peak power output.  I would expect there to be a difference of 27% in power delivery. This is nearly true as the power output is 31% higher.

Where does the gas engine get the extra power from?

The gas engine will be able to produce more power as it is able to burn more fuel per second. If we take a look at the total amount of air entering the engine, we know that the engine will consume 1.8L of air for every 2 rotations of the output shaft. This would equate to 5400 Litres of air per minute on the 130hp engine and  the 170hp engine will consume 6840 Litres of air. More air means more fuel is burned.

Comparing 2 Toyota Engines that have the same displacement but different power outputs.

Comparing 2 Toyota Engines that have the same displacement but different power outputs. 2ZZ-GE left, 1ZZ-FE right

Brushless Motor Comparison

Would you expect this same fundamental to be true in a brushless motor? The short answer is absolutely. Higher Output RPM in both Brushless motors and Combustion Engines help us to achieve more output power.

If we take the same brushless motor, apply the same input voltage  but change the Kv of the motor, we can expect different results. The higher Kv motor will be able to output more power utilizing the same formula as above. This power electrically comes from the higher Kv motor requiring more current to spin higher RPM’s. Electrical power is determine from the formula – Power = Voltage x Current.

Performance for Different Engine / Motor Sizes

Combustion Engines:

This is such a great comparison to look at. For the example in this comparison, we will use a Ford Mustang and a Ferrari 458 Italia. The Ford Mustang in 2015 produces 435 horsepower at 6500 RPM from it’s 5.0L engine. A 2012 Ferrari 458 Italia is able to produce 570 horsepower at 9000 RPM from a 4.5L engine.  The smaller engine from Ferrari is able to out produce the Mustang quite easily. This can be very common with combustion engines, where size of the engine really doesn’t tell you the entire picture in terms of power output. Running the same calculation as above, the mustang consumes 16 250 litres of air per minute, while the 458 Italia consumes 20 250 litres of air per minute. Yet again, you can see that the engine with the higher amount of horsepower is consuming a lot more air per minute.

The 5 litre engine has a 11% advantage in size vs the 4.5L engine. However, the 4.5L engine has a 38% advantage in output RPM equating to a theoretically simplified 25% overall gain in power. The actual power difference between the engines is about 30%.

Popular Turbo vs Size Comparison

To take it a step further the 3.5L V6 that can be found in the Ford Raptor is able to produce 450hp at 5000 RPM. The big difference here is that this V6 has a turbo that pushes 16 pounds of boost down the throat of the engines intake manifold. That is where the power comes from.  A 3.5L is able to produce more power than the 5.0L found in the Mustang even when it is spinning 1500 RPM slower!

Boost pressure is what allows more air to enter the engine. For every 14.7psi of boost pressure, you are getting another atmosphere of air into the engine. At 16 pounds of boost, this is equivalent to 1.089 atmospheres more.

Performing a similar theoretical calculation as above, the 3.5L engine consumes about 18 000 litres of air per minute. This is of course higher than the consumption rate of the Mustang.

Car Engine Size HP RPM Boost Absolute Air per min (litres) Displacement Advantage RPM Advantage Boost Advantage Theoretical HP
Mustang 5.0 435 6500 14.7 16250 1.00 1.00 1.000 435
458 Italia 4.5 570 9000 14.7 20250 0.90 1.38 1.00 542
Raptor 3.5 450 5000 30.7 18274 0.70 0.77 2.09 489

Brushless Motor vs Physical Size and Power Output

In a brushless motor the same performance fundamental can not be expected. A smaller brushless motor spinning at higher RPM will not necessarily be able to keep up in power output as a larger motor. Even if the larger motor is spinning at a slower rate.

The big difference here is that an electric motor requires the physical size difference to be able to fill more motor space with copper. Copper being the windings that are found on the stator. Optimizing the space for more windings allows a motor manufacture to increase the gauge of wire being used, allowing more current to pass through the motor. More current equates to more power. I suppose the saying there’s no replacement for displacement is not so true anymore for ICE and rather more applies to brushless motors. I would gladly take a larger brushless motor when ever I can if I’m looking for more power.

Changing Timing vs Power Output

Combustion Engines

Combustion engines have spark plugs that control the explosion timing of the gases in the combustion chamber. In addition, they also have valves that control the timing of air and fuel entering the engine or exhaust exiting the engine. Changing  ignition timing or even valve timing is able to increase power delivery. Not only can power be increased, overall power delivery can be optimized and smoothed.

Brushless Motors

In fact this same principle is true for Brushless motors. The electronic speed control is typically what is used to control timing. Brushless speed controls are able to control the timing electrically. Feedback received by the ESC allows timing changes continuously depending on load, speed and other factors.

How Load Effects Performance

Combustion Engines

Combustion engines are able to have horsepower specifications given at specific RPM intervals. Knowing what the values are are important in order to understand the potential without testing. What is interesting or uninteresting about combustion engines, is that as you increase the load on these engines, the power delivery will not increase. Combustion engines struggle to produce more power when overloaded. This is why it’s important to load an internal combustion engine correctly so that power is optimized.  It is uncommon for a combustion engine to have any power output specification in terms of peak vs continuous output.

Brushless Motors

Brushless motor manufactures typically do not place power outputs in data sheets. However, if they do, they must or certainly should specify whether the value is a continuous rating or a peak rating. Brushless motors operate entirely different then combustion engines. The biggest difference is if you place additional load on a brushless motor, that motor will do its best to push that load. The problem comes when you realize it was not fit for the extra load, resulting in a burned up motor. This is not an example of mechanically stressing the motor parts as you can easily do this in both brushless motors and combustion engines. it is an example of stressing the electrical windings within the brushless motor.

It’s important to understand this as it is very easy to overload the brushless motor and think all is well.

 

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