smart car

[beef_bourito]

I was just looking up the smart fortwo specs and features, cool car and stuff but I noticed something, it only has 18:1 compression and 1.15bar of boost. Wouldn't higher compression and higher boost give you more power and more mpg? Isn't fuel economy what the smart car is all about? wouldn't it make sense to increase the boost and compression? please fill me in. I'm really liking this car because it's cheap and efficient and it would be great to see possible improvements on the car that could make it better in the areas that it's trying to sell itself in.

[curtis73]

It would increase NOx emissions beyond acceptable levels for this application, and it would increase power, but most likely increase BSFC which may cause MPGs to drop.

[beef_bourito]

what's bsfc and how does it affect mpg?

[ales]

Brake Specific Fuel Consumption (amount of fuel consumed by an engine divided by its power output)

[curtis73]

First of all, I found a great article at EPA.gov. It is the results of their study on Biodiesel. Promising results if we can get those NOx down.

http://www.epa.gov/otaq/models/analy...dsl/p02001.pdf

BSFC stands for Brake Specific Fuel Consumption. I found this definition at another website:

Refers to the efficiency of fuel consumption, per pound, per horsepower produced. The values need to include WEIGHT and TIME per Horsepower. It is acknowledged that 0.42 lbs//Hr. is a standard reference point per Horsepower. At 6.0 lbs per gallon of fuel, a given engine will burn 0.07 GALLONS of fuel per horsepower, per hour. At 400 Horsepower, that would be: 400 x 0.42= 168 lbs / 6 lbs (per gallon)= 28 Gallons/hr. Air density, humidity and temperature outside of standard conditions will vary this mean value

I don't like that definition since the opening sentence makes it sound like another way to measure MPGs. Basically, BSFC measures how much fuel (by weight) your engine requires to make its hp. It CAN affect MPGs and often does, however its important to note that engines only make their given HP at WOT @ X rpm. Actual MPGs happen as a result of 99% of your driving being at 1/4-1/2 throttle. Increasing measured BSFC doesn't always increase BSFC across the board anymore than having a 400-hp engine means you have 400 hp across the board. When you increase static compression (compression ratio) the reason burns become more efficient is because the net pressures in the cylinders are raised. Since upping the compression ratio affects so many other flow characteristics, it tends to not increase cylinder pressures appreciably in other parts of the rpm band and at modest thottles depending on cam characteristics.

In general, increasing compression will improve peak BSFC, but often times reduces it in the mid-RPM range. This is especially holds true of carbureted gas engines. Simply increasing compression will increase the potential for better BSFC, but only if you tune the carb to match it. By altering the compression ratio, you also alter the VE (volumetric efficiency, or how well the engine ingests air) and therefore the airflow characteristics through the carb. It is important to note that BSFC is a measurement of how much fuel is required to support that hp, not how much is actually consumed. Hence the "brake specific" nomenclature. If an engine requires 0.42 lb/hr/hp and you give it 0.425 with poor carb tuning, you will be running very rich and lose power and mpgs, even though the BSFC of the combination didn't change.

Where people get confused is that increasing compression increases the efficiency with which the fuel burns. It gets more energy out of the same charge than lower compression. Although this can equate to better (lower) BSFC, the engine also makes more power which could negate the lower fuel ingestion required. Raising compression gets to a point of diminishing returns. It is after this point of diminishing return that an increase in compression can cause an increased (worse) BSFC.

Here's how it all goes together to answer your question... 1) Raising compression may or may not decrease BSFC depending on cam overlap, intake restrictions, head design, how efficient the engine is already, spark plug location in the head, atmospheric conditions, humidity, and moon phase. Well, not really moon phase I just wanted to make sure you were awake. 2) A decrease in BSFC may or may not equate to better MPGs. BSFC is measured or estimated on the dyno, which is always a WOT (wide open throttle) run from about 2000 rpms up to redline or just past it. What happens to BSFC in the lower and middle RPM bands is pot luck based on your combination. 3) a decrease in BSFC may or may not effect an increase in MPGs. In short, MPG is basically lbs of fuel per mile that is actually pumped through the engine; regardless of what is burned, leaked, wasted, or evaporated. BSFC is pounds of fuel required to make one HP based on your engine combination. By definition, BSFC is lbs/hr and MPGs can be expressed as lb/mile. Two vastly different possibilities.

Now to add more confusion to the pie, even if you optimized your compression (and the rest of your engine) for best BSFC and best MPGs, if you put me behind the wheel, chances are you'll get much worse MPGs than before. I tend to use the extra power you just gave me and I spend more time at WOT than most people. Driving habits are the most difficult to map and the greatest factor in MPGs.

Now here's where Curtis gets to eat his words. I had missed the part that the SmartCar is a diesel. Since diesels have no throttle, always run lean, and rarely do you change its very mild cam, the above properties are often improved with increases in compression or boost (which in effect is just raising dynamic compression). In diesels, when you increase compression you almost always increase pressure across the board, which decreases BSFC. It also decreases BSFC at a faster rate than it increases HP which leads to an overall drop in BSFC. Since the fuel injected is managed by the computer or a mechanical pump, you haven't actively increased the amount of fuel being injected, so MPGs should increase if driving habits stay the same.

The main problem with diesels and increased pressures (or any internal combustion engine for that matter) is NOx emissions. So far we've been able to keep diesel engines NOx acceptable by using more sophisticated EGR systems, but I think the 1.15 bar and 18:1 compression in the SmartCar is a trade off for SULEV (super ultra low emissions vehicle) status. They could go with just ULEV and crank it up to 1.4 bar or so, but I think they made a marketing trade-off.

I hope that didn't confuse. I've been wanting to post that info for quite sometime now. There have been some threads about increasing compression to get better MPGs and I wanted to clear the air about what actually happens dynamically. Compression, BSFC, and MPGs are linked, but not necessarily always in the same way.