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Qwestrum Engineering360 · Automotive & Manufacturing · Transmission Systems

Transmission Efficiency

Transmission efficiency quantifies how much engine power reaches wheels after mechanical and fluid losses.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Bearing and seal losses add parasitic drag
  • Oil viscosity and level affect churning loss
  • CVT efficiency lower than fixed ratio at high torque

Topic details

Introduction

In powertrain matching, B.Tech students must include transmission losses when moving from engine map to wheel tractive effort. Bosch calibration notes and Gillespie vehicle models both use efficiency maps instead of single constant values for higher accuracy.

Key relations & formulas

Formulas (Indian textbook notation)

  • ηtrans=PoutPin×100\eta_{trans} = \frac{P_{out}}{P_{in}} \times 100%
powerloss=T×ω×power loss = T \times \omega \times
(1−η)

Formulas (Indian textbook notation)

  • eachgearmesh 1−2each gear mesh ~1-2% loss typical

Notation and sign conventions

Relation 1 —
ηtrans=PoutPin×100\eta_{trans} = \frac{P_{out}}{P_{in}} \times 100%

Formulas (Indian textbook notation)

  • ηtrans=PoutPin×100\eta_{trans} = \frac{P_{out}}{P_{in}} \times 100%
Write this relation with symbols exactly as in Automobile Engineering — Kirpal Singh before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
powerloss=T×ω×power loss = T \times \omega \times
powerloss=T×ω×power loss = T \times \omega \times
(1−η)
Write this relation with symbols exactly as in Automobile Engineering — Kirpal Singh before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
eachgearmesh 1−2each gear mesh ~1-2% loss typical

Formulas (Indian textbook notation)

  • eachgearmesh 1−2each gear mesh ~1-2% loss typical
Write this relation with symbols exactly as in Automobile Engineering — Kirpal Singh before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Concept in depth

Losses arise from gear mesh friction, bearing drag, seal friction, and lubricant churning, each varying with load, speed, and temperature. Efficiency penalties influence fuel economy calculations and can alter gradeability and acceleration predictions in vehicle simulations.

Assumptions and validity limits

State assumptions explicitly before using any relation for transmission efficiency — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Transmission Systems viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Transmission Systems papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to transmission efficiency.
4. Use equation 1:
ηtrans=PoutPin×100\eta_{trans} = \frac{P_{out}}{P_{in}} \times 100%
.
5. Use equation 2:
powerloss=T×ω×power loss = T \times \omega \times
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Transmission Efficiency appears in passenger and commercial vehicles. In Indian automotive curricula this topic is tested because it connects theory to clutch, gearbox, and differential.
GATE and semester exams often combine transmission efficiency with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use transmission efficiency?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

A common slip is applying percentage efficiency directly as decimal without conversion. Students also compute wheel power from engine torque and wheel speed without accounting for intermediate transmission ratio and efficiency.

Quick revision checklist

Before attempting transmission efficiency problems, confirm you can:
1. Bearing and seal losses add parasitic drag
2. Oil viscosity and level affect churning loss
3. CVT efficiency lower than fixed ratio at high torque
Revise the solved examples in Automobile Engineering — Kirpal Singh and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Power loss estimate

Problem

Input power is 60 kW and transmission efficiency is 92%. Compute output power and loss.

Solution

P_out = 0.92 × 60 = 55.2 kW. Loss = 60 − 55.2 = 4.8 kW.

Conceptual check — Transmission Efficiency

Problem

In a Transmission Systems semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of transmission efficiency." What should a complete answer include?

📖 Standard books (India)

  • Automobile Engineering — Kirpal Singh

    Read: Syllabus unit

    Vehicle layout, transmission, and engines

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