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Old 06-26-2011, 06:41 PM   #186 (permalink)
bwilson4web
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Quote:
Originally Posted by oldbeaver View Post
Hello, these chart must be very useful, I guess. Can you please give us all a small intro to understand what are they and what is their importance?
I will share my understanding in the hope that others may provide more details.

The Brake Specific Fuel Consumption measures how much power is provided versus fuel consumption of an engine in a test stand with the throttle wide open. The engine rpm is controlled by how much resistive force is applied. There are no accessory loads, not even water pump. It is a map of the best-case engine performance.

This chart can be used to find the optimum, rpm-power bands. In theory and practice, one could drive to stay in the peak efficiency band and maximize fuel economy. But a lucky few can do in-car measurements.

To do an in-car measurement, one needs:
  • engine torque
  • engine rpm
  • fuel consumption
Usually torque is the problem but a few cars have them.

The product of engine torque and rpm is the power. Fuel consumption can be measured by mass air flow (14.7 to 1 air to fuel ratio) or fuel injector timing adjusted for the 'dead time.'

Once the data is collected and plotted, we can find inflection points, the knee or sometimes curve in the graph that identifies the optimum engine operating range(s). For example:
  • 0-<980 - engine off, hybrid mode, very good
  • 980-1200 - engine idle during warm-up, avoid
  • 1200-2400 - good place to be
  • 2400-3200 - mileage starts to decline
  • 3200-4150 - owch
  • 4150-4500 - do not go there
These are specific to the 1.5L, NHW11 model Prius.

Bob Wilson
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2019 Tesla Model 3 Std. Range Plus - 215 mi EV
2017 BMW i3-REx - 106 mi EV, 88 mi mid-grade
Retired engineer, Huntsville, AL
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