Kansas Wrap Up: What Caused all the Engine Failures?

 Andy Randolph, Engines, Kansas, rear-end gear, transmission  5 Responses »
Apr 262012
 

The defining characteristic of the Kansas race was the surprising number of engine problems.  Many of those problems can be attributed to the change in rear gear from a 3.89 to a 4.00.  At  190 mph at a track like Kansas, your wheels make 2270 revolutions per minute (rpm).  If you watch the telemetry on the television broadcast, you know that the engine is rotating around 9500-9900 rpm.  Since the engine is attached to the wheels, there has to be something to change the rotation rate between the engine and the gears.

Gearing Up

That something is the transmission and the rear gear.  As shown at right (with the values given for a Corvette ZR-1), the engine rotation passes through the transmission and then through the rear-end gear before reaching the wheels.  A 4.00 gear means that the ratio of rpm in to rpm out is 4.00:1.  It takes four revolutions of the input to produce one revolution on the output.  If you have something rotating at 8000 rpm and you add a 4.00 gear, then the rotation is reduced to 8,000 rpm/4.00 = 2,000 rpm.

Note that NASCAR does not allow 5th or 6th gears and does not allow overdrive (when the first number is smaller than the second).  The lowest gear you can have is 1:1 in NASCAR.

Let’s compare running at 190 mph with the two different gears.  Last year, a 3.89 gear was used. At 180 mph, you’d better be running in 4th gear (which means 1:1 and the speed coming into the rear end gear is the same as that coming from the engine.  The engine speed required to go 190 mph is this 3.89*2270 rpm = 8830 rpm.  This year, with a 4.00 gear, you’d need to be running at 9080.  If you’re running 200 mph, last year you needed 9293 rpm and this year it would be up around 9556 rpm.  You’re basically running 250 rpm (or so) higher this year than last year at the same speed.

Andy Randolph, Engine Technical Director at ECR Engines tells me that engines were running at 9800 rpm for sustained times.  Although the engine rotates that fast at some places, doing it continuously places huge stress on the mechanical parts – that’s why most of the failures were due to mechanical breakage.  (Because I know he’s too modest to mention it, I’ll point out that none of the engines that had problems at Kansas were from ECR.)

The Math

For those of you wondering about where my numbers come from, here’s a calculation I did for Las Vegas.  The only difference is the slight variation in tire circumference.  If you plug in the numbers to the formulas and don’t get what I got above, I probably screwed up on the calculator.

Left-side and right-side tires have difference circumferences.  The circumference of a left-side Vegas tire in 2008 was 87.4″, while the right-side tires had a circumference of 88.7″.

To calculate how many times the tires rotate each minute, I first convert the speed into inches per minutes.  I know to use those units because I’m trying to get an answer in revolutions per minute, so I need to convert hours to minutes. I also know that every time a right-side tire makes one complete rotation, it has traveled 88.7 inches, so I’m going to convert miles to inches because I know I will need that later. Convert 45 mph to inches:

45 mph corresponds to 47,520 inches per minute. Looking at the right-side tires (for no particular reason), the car travels 88.7 inches every time it makes one full rotation. The number of times the tires rotate each minute is 536 rpm, as shown below.

 

 

 

Kansas: Temperature and Horsepower

 Andy Randolph, Electronic Fuel Injection, Engines, Uncategorized  3 Responses »
Apr 232012
 

There were a lot of engine problems at the Kansas race last Sunday — and a lot of theories as to why there were a lot of engine problems.  Let’s start with the cooler-than-expected temperatures on Sunday.

When the air temperature changes, so does the number of air molecules heading into the engine.  Colder temperatures make air more dense.  Since density is the ratio of mass per unit volume, a volume of air at a lower temperature contains more molecules than the same volume of air at a higher temperature.  The plot below shows how air density changes between 0 and 100 °Fahrenheit.

Before EFI, changes in temperature during a race posed a problem.  Fuel and air prefer to combust with a very particular ratio that is determined by stoichiometry.  Remember all the balancing equations you did in chemistry?  It’s the same thing.

Combusting two octane (a component of gasoline) molecules require 25 oxygen molecules.   The ideal air:fuel ratio is 14.7:1.  If you have one ounce of gasoline, you would need 14.7 ounces of air.  NASCAR engines run slightly richer (meaning a smaller air:fuel ratio).

Engines introduce a fixed volume of air, which means that the number of air molecules changes depending on the density of the air.  You would like a system that introduces exactly the right number of gasoline molecules for the amount of air being introduced.  A carburetor cannot automatically adjust itself to maintain that ratio, but the NASCAR EFI system can.  When the temperature at Kansas turned out to be 20 °F cooler than expected, engine tuners weren’t worried because the EFI automatically compensates for the changing in temperature.

In fact, cooler is better in terms of horsepower production.  The more oxygen molecules in the cylinder, the more gasoline you can inject and the more power you can make.  That’s the idea behind turbochargers – compress the air so that you have more oxygen molecules in a volume of air.

The change in horsepower depends on the square root of the absolute temperature.  You may remember absolute temperature from chemistry and/or physics class.  When you use the ideal gas law, for example, you can’t just plug in the temperature you read from the thermometer.

The Fahrenheit and Celsius scales were developed around things we experience every day.  Water freezing is 0°C or 32 °F.  Water boiling in 212°F or 100°C.  As we discovered more about the molecular nature of temperature, we learned that physics places limits on how cold something can be.  The coldest possible temperature corresponds to -459.67 degrees Fahrenheit.  Rounding that to -460 °F for simplicity, 0 °F is 460 on the absolute temperature scale.  You get the absolute temperature by adding 460 °F to the temperature from the thermometer.  A temperature of 57 degrees F would be (460+57=)  517 F on an absolute temperature scale.  A temperature of 77 F would be 537 F.

The change in horsepower is proportional to the inverse square root of the ratio of the two temperatures.

If I go from 77 °F (537 °F in absolute scale) to 57 °F (517 °F in absolute scale), the horsepower would be:

This represents a 1.9% increase in horsepower.  If the engine was producing 850 hp at 77 °F, it would produce 866 hp at 57 °F.  In a sport where engine builders working really hard to get 1 or 2 hp, this is a huge change!

Some people have suggested that the engine failures at Kansas were due to the increased horsepower produced by the colder temperatures.  My favorite engine technical director Andy Randolph (of ECR engines) tells me this isn’t the likely cause for the engine failures.

What IS the cause will be my next post. (And it’s not EFI!)