Two Guys Rally

Sequential Transmission and Rally America: An Update [ March 31st, 2009 ] By:Charles Smith

In June of 2008 I wrote an article lamenting about Rally America’s rules on sequential transmissions. Turns out they’ve caved…a bit. I was reading the rules and I stumbled over this on page 89 section 6c:

Gear Change: The use of an unassisted manual sequential gear change mechanism is allowed.

Neat! You can use a sequential given that you have direct access to the mechanical selector. That is only one step away from an electro-whathaveyou sequential with paddle controls. Although you cannot control many parts electronically (yet), as seen on page 89 section 6b:

Electronic Controls: No type or form of electronic control is permitted for the following components:
i. Suspension, steering, braking, gear change/clutch, front and rear differentials.
ii. Simple engine cut operating during a mechanically activated gear change is permitted.

So you cannot control the clutch/transmission electronically yet. But you can flat shift, which means you keep your foot planted on the gas pedal during shifts and electronically cut the engine’s power. Either way, there is hope for Rally America to become the premiere rally league in the future. Not for a while though.

Combustion Engine Theory: Intro [ March 29th, 2009 ] By:Mark Ozimek

I was thinking the other day (rare, I know) it’s about time that I started up another interesting series. How about something that is at the heart and soul of almost every motorsport: The engine!

This is just the introduction to engines, I’ll cover the common terminology and give everyone a good starting point for understanding the finer design choices that I will cover later on.

Most automotive racing is powered by internal combustion engines. There are some variations within there to make things interesting. There are usually either two stroke or four stroke engines. I’m gonna focus on the latter, because two stroke engines are basically non-existent in rally. In four stroke engines, there are… wait for it… four strokes!

• Intake: The piston is moving downward, away from Top Dead Center (TDC) with the intake valve(s) open and the exhaust valve(s) closed. This draws in fresh air/fuel mixture to burn later.
• Compression: Intake valve closes, usually just after Bottom Dead Center (BDC), and the piston moves up to compress the air/fuel mix. Just before TDC (usually 10º-40º of crankshaft rotation), the combustion begins, either by igniting the fuel with a spark (gasoline) or injecting the fuel into the compressed air (diesel)
• Expansion: This is the stroke where the power comes from, the burning air/fuel mix generates a lot of heat and pressure that pushes the piston down, generating torque.
• Exhaust: Just before BDC, the exhaust valve opens up, and the piston moves back up to push all of the burnt gasses out of the engine. Once it reaches TDC, things start over again with the intake stroke.

The next choice is the type of fuel. Diesel engines run by compressing air a lot to generate very high temperatures, then inject the fuel, which combusts to generate pressure and heat that drives the engine. Spark ignition engines run a bunch of different fuels, usually gasoline, but can also include mixes of gasoline, ethanol, methanol, propane, compressed natural gas, and a few others. The air fuel mixture is ignited with a spark instead of relying on the sheer amount of heat in the diesel cycle.

There are also variations of the typical reciprocating piston engine, the most common being a rotary, or Wankel engine. The piston is replaced by a rotor with three faces housed inside of an oval-like housing that is technically known as an epitrochoid. Rotary engines have a very high power/displacement ratio because there are three power strokes for every revolution of the rotor, compared to the one power stroke every two revolutions of the crankshaft of a normal four stroke piston engine. However, sealing and lubricating the piston is a significant issue that hampers the reliability of this design

So now that this is out of the way, it’s time to get into the common terms used when describing or talking about engines. Let’s start at the macro level, things that everyone should be familiar with and move our way in.

Displacement: The amount of volume that all of the pistons displace in one stoke. This is dependent upon the number of cylinders, the bore and stroke.

Bore: The diameter of the cylinder when viewed from the top. The piston diameter is a small amount less than this, with rings to provide a good seal

Stroke: The distance the piston moves from TDC to BDC. This is dependent upon the crankshaft dimensions.

Compression Ratio: The ratio between the displaced volume (Vd) and the volume in the top of the cylinder (Vc) when the piston is at TDC. To calculate, CR = (Vd + Vc) / Vc.

Air/Fuel Ratio: Also known as AFR, or it’s reciprocal, FAR. Gasoline likes to burn within a specific range of ratios between the mass of air present and the mass of fuel present, typically between 8:1 to 20:1. the combustion can be considered the most “complete” when the AFR is stoichiometric (the wiki article does a better job explaining the chemistry than I ever could), 14.7:1 for pure gasoline, or ~14.2:1 for the 10% ethanol blend that almost all pump gas is now. This means that for every 14.7 kilograms of air that flows through the engine, the engine will try to supply 1 kg of gasoline. Ratios that are lower than stoich are called “rich”, and higher is “lean”. Given a constant set of parameters and optimized ignition advance, AFRs around 12.5-13 for gasoline give the most torque, because the fuel burns the fastest then.

Ignition Advance: Measured in the number of crankshaft degrees before the piston reaches TDC. Typically spark will be tuned to create maximum cylinder pressure around 14º after TDC. More advance is needed when the engine spins faster, because the burn speed of gasoline does not increase with the engine speed. However, the burn speed does increase with air density, and with AFR, with a maximum burn speed for gasoline being around 12.5-13. As such, timing is typically less advanced with more open throttle or higher boost pressure, but more advanced at higher engine speeds. Many design factors play a role in optimal ignition timing.

Volumetric Efficiency: This is essentially a measure of the amount of air that goes into a cylinder compared to how much a piston displaces. Since air is compressible, meaning the density changes with pressure, it makes more sense to think of it in terms of mass. A volumetric efficiency of 100% would imply that the mass of air that is in a piston is the same as the mass of whatever the displacement would weigh in the surrounding air. So taking the 100% efficiency example further, if it was a 4 cylinder 2.0L engine running at 100% efficiency at STP, the mass of air inside one cylinder would be equal to the density of air (1.184 kg/m³) multiplied by the volume (0.5L), the result is 0.592 grams of air. Doesn’t sound like a lot, but air is pretty light, and when you’re turning the engine at 6000 rpm, the engine is moving about 7 kg/min of air.

Mean Piston Speed: The average speed of the piston as it moves through a cycle. This is dependent upon the RPM (referred to as N in calculations) that the engine is running at and the stroke. To calculate, Sp = 2 * N * Stroke. Due to material strength and fatigue limitations, it is uncommon to see the mean piston speed exceed 25m/s or so, except in extremely high performance racing engines, like F1.

Brake Mean Effective Pressure: Commonly BMEP (or MEP when not measured at peak torque or power), this a way to measure how effective an engine is at making power in relation to it’s displacement and rpm. As a general rule of thumb, the more power you make per amount of displacement and the less rotational speed at that power level, the higher the BMEP is. Alternately, for those of you who know how torque, power and rpm relate to each other, the peak BMEP of the engine is at the peak torque of the engine. To calculate MEP, you need to know either the power and RPM, or torque, displacement, and number of strokes (2 or 4)

Calculating with power and rpm:

MEP = (P * Nr * C) / (Vd * N)

P is power, in HP or kW
Nr is the number of revolutions per power stroke, 1 for 2-stroke, or 2 for 4-stroke
C is a constant, use 396,000 for imperial (hp & ft-lbs) or 10³ for SI (kW & N-m)
Vd is displacement, cubic inches for imperial or liters for SI (61.02 CI per L if you need to convert)
N is the engine speed in RPM

Or with torque:

MEP = (T * C) / (Vd)

T is torque, ft-lbs or N-M
C is a constant, 75.4 for imperial, 6.28 for SI

Well, that’s it for the intro. I’m sure some of you have specific things that you would like me to go into detail on in this series, feel free to ask, and I’ll try my best to cover it!

Brake Dust vs Elbow Grease [ March 18th, 2009 ] By:Mark Ozimek

I was about to put my summer tires back on the Volvo today, but when I pulled the wheels out of the closet, I realized how dirty they are. I used the rain as an excuse to spend some time getting off some of the grime that had built up over the last 11 years and 150,000 miles!

Last fall, when I put the snow tires on, I tried to scrub down the inside of the wheel, and got a bunch of stuff off, but it still looked horrible:

So I made a quick run to a Pepboys down the road and picked up some Meguiar’s Hot Rim cleaning spray. It says to just spray on and rinse off. That’s a blatant lie. I effectively destroyed a sponge scrubbing all the crap off the surface. The spray really did help break down the baked on brake dust, and after rinsing everything off, that wheel now looks like this:

Much better! Not bad for about 10 minutes worth of scrubbing… hahaha. However, not perfect because quite a few of those black spots aren’t dirt, but damage to the surface of the wheel.

So why do this? Well, obviously it look nicer now. More practically, cleaning stuff off the surface prevents oxidation from occuring, and if there is too much dirt and grime, the balance of the wheel can be thrown off, requiring the wheel to be rebalanced to reduce vibration at high speeds.

Rally Cyprus 2009 - YAY! [ March 16th, 2009 ] By:Charles Smith

It is the first time in a long time that a private (not really factory supported) team has podiumed in the WRC’s top class. While we’re stuck with two manufacturers, there are still plenty of teams. One of my favorite teams is Petter Solberg’s self run rally team and he happened to finish 3rd in Rally Cyprus. That really is indicative of the talent Petter Solberg has and it really makes me wonder about Subaru. Maybe there really was something wrong with how Prodrive was making the Subaru WRC car. 

In usual fashion Sebastien Loeb finished first. I don’t know how he keeps that up. Many people would lose focus after winning as much as he has. He keeps getting faster too. It really is quite amazing.

Mikko Hirvonen, Ford’s favorite for the WRC pulled out a 2nd place finish. He’s now only 8 points behind Loeb in the championship standings and with 75% of the season’s points still up for grabs he has plenty of hope to be the World Rally Champion. 

The next WRC race is April 2nd, 2009 and it is Rally Portugal.