Elevation and the Combustion Engine

Elevation and the Combustion Engine

Your Colorado car may not need a tune-up after all…

Picture yourself on a road trip from Chicago to Denver, West-bound on I-80 with the cruise control set at 75.  You’ve just stopped for sleep and fuel in beautiful Kearney, Nebraska.  This “midwest treasure” is home to the most picturesque Motel 6 you might ever see and a Sinclair station that, yes, still has the famed green dinosaur out front.  As you pull out of Sinclair and goose it back on to the highway though, you notice something; Your trusty chariot seems down on power, and not just by a little bit.  Was it the fact that you chose to fund the proprietors of the green dinosaur rather than the Shell station across the street?  Was it the fact that you ate those three extra dinner rolls at the Cracker Barrel last night?  The answer resides in neither a fuel quality or carbohydrate-based excuse, but rather in one simple word: Elevation.

 Kearney, Nebraska rides the almost exact geographical mid-point on I-80 between Iowa and Colorado.  At 2,131ft, Kearney is the first major city on this Chicago-to-Denver route whose elevation surpasses 2,000 feet above sea level.  Grand Island, which is about 45 minutes East, sits at 1,856.  Lexington, which is about 45 minutes West, sits at a towering 2,408.  Although the surrounding land seems deceptively flat and full of corn, the clay underneath you is actually rising at a fairly rapid rate towards the sky.  As a result, the air becomes less saturated with oxygen and, in turn, your engine has to work much harder to create a complete and efficient combustion cycle.

As you arrive in Colorado, you notice that even more of your car’s precious acceleration (and fuel economy) have gone the way of the dinosaur.  Denver sits nearly 3,000 feet above Kearney at 5,280, exactly one mile above the sea.  According to http://www.ifsja.org, the formula for horsepower loss due to altitude is ‘elevation x 0.03 x horsepower at sea level.’  In other words, your 300-horsepower beast (at low altitude) has lost 47.52 horsepower simply from making the trek to the Mile-High City.  Granted, there are other natural parasitic losses that lower the 300-horsepower rating right from the factory.  Though your engine may produce 300 horsepower on its own, there is much energy lost when that power is routed through a flywheel, transmission, differential(s), driveshaft and axles.  When a car is dyno-tested, or has its power measured at the wheels rather than at the engine, this is why the power output number is significantly lower.  For example, when mustang50magazine.com dyno-tested a 2005 Ford Mustang GT (rated at 300 horsepower), it only produced 242.9 ‘useable’ horsepower at the rear wheels.  Utilizing the aforementioned formula for power loss due to elevation, if this Mustang were driven in Denver (5,280ft), it would be putting down something more like 195.38 rear-wheel horsepower.

At this point, a Denver resident might ask where the bright spot resides in this article, and the answer is simple.

When seeking a new or used vehicle in a high-altitude market, look for a vehicle that is turbocharged.

The debate over whether or not ‘forced-induction’ (turbocharged or supercharged) vehicles are statistically more efficient than ‘naturally-aspirated’ engines at high altitude remains mostly unsolved.  One consistent truth behind this, though, is turbocharging is more efficient than supercharging.  The theory behind supercharged engines is that at high altitude, they are no more efficient than a naturally-aspirated engine.  This thought stems from the fact that a supercharger is belt-driven, much like an alternator, power steering pump or air conditioning compressor.  As such, it utilizes the engine’s power to turn a pulley and belt system which, in turn, winds up the insides of the supercharger to ‘force’ extra air down into the engine for combustion.  The advantage of this process is the virtual elimination of ‘lag’ when accelerating, due to the fact that a supercharger’s insides are turning efficiently even when the engine is idling.

Concurrently, with turbocharging, the engine’s expelled hot exhaust gases are forced through a turbine-spindle-compressor assembly (turbocharger).  This process happens even before the exhaust has a chance to escape through the car’s mufflers, catalytic converters and tailpipes.  As such, the high velocity of the exhaust gases from the engine are blown through the turbine’s ‘blades,’ ‘allowing the turbine to ‘spool up.’  This then turns the attaching spindle, which is connected to the compressor and its own respective ‘fans.’   Once spooled, the compressor’s blades suck fresh air upwards and accelerate it back into the engine.  The turbine’s blades, in turn, force the already-used hot exhaust gases back into the exhaust stream and out into the atmosphere.

The downside to this system is that the engine has to complete an entire combustion cycle, wait for the turbine and compressor to spool and then wait for the fresh air to be recirculated all the way back to the engine’s intake to create power.  This is why turbocharged engines experience ‘turbo lag,’ a delay experienced mostly during acceleration from a standstill.  Another downside is that this system creates a great amount of heat and, as such, can be considered thermally inefficient.  This is because hot air is less oxygen-rich than cold air and, in turn, cannot be combusted by your engine as efficiently.  As a result, especially in warm ambient temperatures or in stop-and-go driving, the compressor is typically forcing hot air back into the engine.  Manufacturers have recognized this issue and began using ‘intercoolers’ to aid the process.  The intercooler typically sits in the middle of the piping between the turbocharger’s compressor and your engine’s air intake.  When the compressor expels fresh air back towards the engine, it is first passed through the intercooler which lowers the temperature of this air substantially.

How turbocharging and intercooling benefits you is simple: More power and efficiency are available more frequently and under more varied conditions.  By the turbocharging and intercooling principle, your engine has an ability to keep the air it breathes significantly faster-flowing, cooler, and in turn, oxygen-rich.  As a result, the engine does not necessarily have to work as hard to achieve greater levels of power and efficiency.  Plus, manufacturers have a tendency to ‘under-rate’ the power and efficiency of turbocharged engines from the factory.  For example, Subaru rates the horsepower of its turbocharged 2011 WRX engine at 265bhp.  Several sources, though, have dyno-tested this engine at over 250 useable horsepower at the wheels.  Taking into account a normal power loss from the aforementioned flywheel, transmission, differentials, driveshaft and axles, this is reflective of the engine most likely developing closer to 300bhp.  This offers further evidence that, although not always consistent, a turbocharged engine’s efficiency can surpass even its own manufacturer’s expectations.

Put simply, the next time you are either road-tripping through or moving to a high-altitude area, remember that your car’s lack of power is not something that can be adjusted or controlled.  Like us humans, today’s fuel-injected and computerized cars can automatically compensate for different oxygen levels in the air.  How the engine is able to mitigate the difference, however, resides within how efficiently it can do so internally.  With a turbocharged engine, there seems to be little question as to the benefits in efficiency it provides at higher altitudes.  Try test-driving a turbocharged Subaru WRX, Audi A4, Acura RDX or BMW X3 next time you’re out shopping for a car.  Trust me, the difference you feel will be well worth it, especially when taking your next trip through the Rockies.

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