They Don’t Call It The “Power Section” For Nothing (part 2)
Fifth Edition, Vol. 1
This month I’m going to continue with the topic from last time. Like the title states, the power section is just that and it consists of more than just the valve train discussed in Part 1. The reason for this continuation is prompted by a discussion I and a few other aviators had with one of our local airplane experts regarding compression(s) and how “they say that good compression doesn’t really make that much difference anymore”. This “expert” wouldn’t name the source of this whowee, but I surmised that it was some “frugal” pilot that thinks he knows more about his airplane than the licensed folks who have been keeping these birds in the air since Chuck Taylor and the Wright brothers first flew off the Kill Devil sand dunes in 1902. During this “experts” testimony to his beliefs, I asked him to explain the science behind this theory and maybe explain how, in layman’s terms, this lack of compression affected the power generated by his engine. He said that “well, that’s what they say…………I don’t know, but it sounds reasonable”. This “expert” must’ve seen the apparent look of utter disagreement on my face, as he continued to condone his beliefs through some hopefulness that the low compression numbers that he must have experienced at his last annual inspection, might be considered acceptable with the revelation of this new information. In my usual tactlessness, I proceeded to inform our resident “expert” that his belief that low compression wasn’t that big of a deal was not only false, but the theory he spoke of was unsubstantiated and unproven not only by the source he obtained this information from, but also from years and years of proven practice, and my own experience. I used the analogy of one’s car cruising down a desert road at a given speed. The vehicle can maintain that speed provided the engine possess some reasonable amount of compression. Now, you approach a hill, (or ascend in altitude in your aircraft. Which introduces another aspect I’ll get to in a minute), if at the same throttle setting your vehicle can maintain a somewhat steady speed without increasing the throttle setting, (maintaining manifold pressure), your engine’s compression is probably very good. But, if you have to press on the accelerator pedal to the floor, (increase manifold pressure), or even down shift to the next lower gear to climb that same hill, then the engine’s compression is low. Low to the point where it cannot produce enough power to overcome the resistance from the gravity of the vehicle.
Years ago, I had the pleasure to become friends with a very well known VW engine guy when I was racing Formula “V”’s. I don’t like to name drop, but for the sake of this discussion I will divulge his identity, but first I will explain the conversation that I had with this industry Icon. Maybe you can guess who this mystery person is before I let the cat out of the bag. This person has “gone west” now, but had been a fixture in the early years of VW drag racing. “Burger” as he was known by most, took a liking to me because I wanted to learn everything I could from him about getting more power out of my 1192 cc VW engine in my FV. He was a drag racer, but going fast even if you had to turn the wheel back and forth, was still racing. Even racing in the dirt was still racing in his eyes, so I was accepted into his inner circle of speed freaks, and felt privileged and honored to be allowed access to this encyclopedia of knowledge. “Burger” was getting up in the years, and he seemed to want someone interested enough to pass on this hard earned knowledge to the next generation, and it seems I was the closest he was going to come to at the time………… I guess.
One of “Burger’s” popular sayings when it came to compression and getting power out of the VW engine, (any internal combustion engine) was this; “Compression = Heat, Heat = Energy, and Energy = Power. You cannot have any two without the third”. This is the exact example I gave our “expert” on his belief that low compression was now considered acceptable in his aircraft engine. This is like “alternative math”. In the case of an aircraft, the thing would have such poor climb rate with low compression, it would be dangerous especially at altitude or any field with a high density altitude. Now, this discussion theory is for naturally aspirated, (N.A.), engines. Supercharged or Turbo Supercharged engines are designed with a lower compression ratios to begin with and maintain cylinder pressures by the use of such blowers, so this topic does not include any “blown” engine designs.
Take for example, the Continental O-470-R engine in the Cessna 182. It’s a N.A. engine of 471 cubic inch displacement (c.i.d.), that produces 230 hp. at 2600 RPM, with a compression ratio (CR) of 7:1. Its cousin, the IO-470-F is a fuel injected version with the same 471 c.i.d., a CR of 8.6:1 and it produces 260 hp. at 2625 RPM. 30 more ponies at only 25 more RPM’s. As a rule of thumb, a gain of roughly ten h.p. per 100 RPM’s is achieved. Fuel Injection will give you about ten more ponies, so the other remaining ponies basically come from the higher CR. Higher Cylinder Head Temperatures are also experienced because of the higher CR. Remember, Compression = Heat, Heat = Energy, Energy = Power. A low compression engine will always run cooler than a high compression engine. The guys who install those big bore kits in the VW engine and want 10:1 pistons, have to add a larger fan or an oil cooler to overcome the excess heat generated beyond what the factory designed for. They’ll also have to run a higher octane fuel in it to prevent detonation also, but that’s a topic for another time.
Another factor in gaining Horse Power (h.p.) is RPM. The faster you turn an engine, the more horse power is achieved. Although torque will drop off at a certain point, h.p. will continue to rise until the efficiency or the mechanical limitation is reached. There was another saying “Burger” had. “If you take horse power, and you give me torque, I’ll beat you every time”. Now, the torque is not much at the top end of the RPM range, but down low, will make the difference getting off the line quickly without too much wheel spin and can be carried through the lower RPM ranges where “hook-up” is most advantages. This is why when you downshift to climb that hill on that desert road, you can maintain somewhat of the same speed. You’ve kind of exchanged the torque at the low RPM to h.p. of the higher RPM. The “power” in a Diesel, (compression ignition) engine is measured in torque, not horsepower. A typical Diesel engine will have a high torque value compared to a low horsepower value at any given RPM. Diesel engines also have a very short power band. This is to exploit the torque band and is the reason Diesel powered vehicles have so many gear ranges. A typical tractor-trailer rig could have 16 speeds available to maintain efficiency of the torque at all driving speeds and conditions. The CR in a Diesel engine is typically from 14:1 to 25:1 depending on the c.i.d. of the engine. These high CR’s is to ignite the fuel in the combustion chamber, but is also directly proportional to the power output derived from the compression itself.
So, in respect to our “expert” and his theory that low compression is not that big of a deal, I say “blödsinn”. His 360 c.i.d., (361 c.i.d. actual) Lycoming puts out 180 h.p. at 2600 RPM. This engine is essentially a Diesel engine with spark plugs. It has a large bore, turns relatively slow, and produces a meager 180 h.p. at that low RPM. The relatively low RPM in a direct drive aircraft engine is to keep the propeller tip speeds from reaching supersonic. The propeller gear reduction in a “geared” engine, converts the higher engine RPM h.p., to low RPM torque. “Burger’s” Stuska dyno used a resistance meter to measure torque, as well as RPM (and fuel flow) to calculate h.p. Some big radial aircraft engines had a simple apparatus mounted on the propeller reduction gear case that measured resistance in the propeller reduction gearing to calculate torque and/or h.p.
Sorry, I wandered a little off topic. Back to our “experts” O-360. All things being equal and in good condition, this engine should generate 363.3 foot pounds (lb/ft.) of torque at 2600 RPM. (180 h.p. X 5,252 = 945,360 / 2,600 RPM = 363.3 lb/ft.). That’s slightly over one lb/ft. of torque per each cubic inch of displacement. That’s pretty damn good in my book. If his engine is worn and the compression is lower than recommended, then the engine cannot produce the needed torque, (363.3 lb/ft.) to produce the needed power, (180 h.p.) at the limited 2600 RPM to meet the published climb numbers of his aircraft. This condition can be dangerous when trying to climb out over that 50 foot obstacle in that back country airstrip when the density altitude is pushing 10,000 feet MSL. To say that low compression numbers are acceptable is ludicrous.
So, to recap. If your engine’s compression is lower than desirable, then your overall “power” will be lower than desirable. A good indicator is a cool running engine no matter what the ambient temperature is, especially when you point it up. If the temps don’t climb with vehicle attitude, then you have a power generation deficiency and it will be evident in the climb rate no matter whether it’s climbing that desert hill, or climbing out of that back country airstrip. We all could use “more power” in everything. It appears our “expert” seems to be reading too many “comic books”, and is easily convinced of “new” theories regarding aircraft maintenance and operation. As ol’ Phil Woodall used to say, “you can fool the fans, but you can’t fool the players”. And I’m sure Lee Leighton would agree.