Most underground mine planners size their diesel equipment fleets based on production requirements and equipment utilization targets. You need to move X tonnes per day, loaders can handle Y tonnes per cycle at Z cycles per hour, so you need N loaders to hit targets with reasonable utilization rates.

This ignores a massive variable cost: ventilation. Every piece of diesel equipment underground requires ventilation to exhaust combustion products. That ventilation costs money—a lot more money than most operations realize.

After reviewing ventilation and equipment operating costs from eight Australian underground mines, I’m convinced most operations are running undersized fleets because they’re not accounting for the ventilation economics properly.

The Ventilation Math

Diesel engines produce combustible gases (CO, NOx, particulates) that must be diluted to safe concentrations. Australian standards (MDG 29) require minimum ventilation rates based on engine power output.

Rule of thumb: 3.0-3.5 cubic meters per second of airflow per 100 kW of installed diesel engine power.

A typical underground loader (5-6 tonne capacity) has roughly 200 kW engine power. That requires 6-7 m³/s of dedicated ventilation airflow.

If you’re running six loaders simultaneously in a production level, that’s 36-42 m³/s just for the loaders. Add light vehicles, trucks, and service equipment and you’re easily at 60-80 m³/s for the diesel fleet.

What That Costs

Ventilation isn’t free. You’re moving massive volumes of air through kilometers of development against friction resistance. That takes fan power.

Based on typical underground ventilation resistance (2.5-4.5 kPa depending on airway length and cross-section), moving 70 m³/s requires roughly 175-315 kW of fan power running continuously.

At $0.18/kWh (typical industrial power rate), that’s $31-56 per hour in electricity costs just to ventilate diesel equipment. Over a 24-hour operating day, that’s $740-1,340 per day for the diesel fleet’s ventilation.

Annually: $270K-490K in power costs solely attributable to diesel equipment ventilation.

And that’s just the energy. Add fan maintenance, ductwork installation and maintenance, and engineering time managing ventilation circuits, and you’re probably at $400K-700K per year in total ventilation costs for a mid-sized diesel fleet.

The Alternative Economics

Now compare to running fewer, larger diesel units. If you replace six 200 kW loaders with four 300 kW loaders, total installed power stays roughly similar (1200 kW vs 1200 kW). But you’ve got four engines instead of six.

Ventilation requirements stay proportional to total power, so no savings there. But you’ve reduced:

• Number of machines requiring maintenance
• Number of operators needed
• Tire consumption (fewer machines)
• Service vehicle requirements

The ventilation cost per tonne of material moved goes down because you’re running the remaining equipment at higher utilization.

Or consider the battery-electric equipment option. I talked to one operation that replaced three diesel loaders with two battery-electric loaders. The battery loaders have higher upfront cost ($950K vs $650K per unit) but zero ventilation requirements.

They calculated ventilation savings at $180K per year for removing three diesel engines from the fleet. Amortized over 10-year equipment life, that’s $1.8M in avoided ventilation costs.

Added to diesel fuel savings ($85K/year) and reduced maintenance costs ($65K/year), the battery loaders had better total cost of ownership despite 45% higher purchase price.

Why Mines Get This Wrong

Most mine planning separates equipment decisions from infrastructure decisions. The mining engineering team sizes the fleet based on production targets. The ventilation engineering team designs ventilation systems to support whatever fleet the mining team specified.

Nobody’s optimizing across both simultaneously. The mining team isn’t penalized for specifying a fleet that requires expensive ventilation. The ventilation team isn’t empowered to push back and suggest fewer, larger units or alternative technologies.

I’ve also seen operations that legacy-planned their fleets when diesel fuel was cheap (sub-$1.20/L in 2020) and ventilation electricity costs were lower. They never revisited those decisions as operating conditions changed.

The result is fleets sized based on outdated assumptions that no longer reflect actual operating economics.

The Staffing Question

There’s another hidden factor: operator availability. Mines struggling to staff full shifts often run undersized crews relative to their equipment fleet.

If you’ve got eight loaders but can only staff six during night shift, you’re carrying capital costs and maintenance overhead for two idle machines. That’s obviously inefficient.

But reducing fleet size creates risk. If a machine breaks down and you’re already running at minimum fleet size, production suffers. So operations maintain excess equipment as redundancy.

The smarter approach is fewer, more capable machines with engineered redundancy. Four high-capacity loaders with good maintenance can deliver the same production as six smaller loaders run hard, and they’re easier to staff.

Real-World Example

I know a zinc mine in Queensland that ran nine diesel loaders (180-200 kW each) across two production levels. They calculated that ventilation for the diesel fleet cost them roughly $520K per year in total (power, maintenance, capital amortization for ventilation infrastructure).

They replaced four of the diesel loaders with three battery-electric units over 18 months. The battery loaders required charging infrastructure investment ($420K) but eliminated ventilation requirements for those units.

Net result after two years of operation:

• Ventilation costs down $210K/year (eliminated 4 diesel engines)
• Diesel fuel costs down $68K/year
• Maintenance costs down $42K/year
• Charging infrastructure amortized over 10 years: $42K/year
• Battery replacement reserves: $35K/year

Annual savings: $243K

They’re planning to replace the remaining diesel loaders as they reach end-of-life. The business case is clear once you account for ventilation economics properly.

Optimization Opportunities

For operations looking at fleet optimization, here’s what actually matters:

1. Calculate total cost of diesel ventilation. Include electricity, fan maintenance, ductwork, and engineering time. Most mines discover this is 2-4X what they assumed.

2. Model fewer, larger equipment. Don’t just match installed power 1:1. Look at whether three 300 kW loaders can deliver what six 150 kW loaders currently produce, factoring in utilization and cycle times.

3. Consider hybrid approaches. Battery-electric for predictable, high-intensity work (production loading). Diesel for flexible, variable work (development, services). Optimize each application separately.

4. Include staffing constraints. If you can’t reliably staff your current fleet, right-size to actual crew availability rather than carrying idle equipment.

5. Model production rate vs. operating cost. Sometimes running fewer machines at slightly lower production rates delivers better economics than maxing out production with high ventilation costs.

What Changes the Calculation

Battery-electric equipment is getting better fast. Range and power limitations that ruled out electric loaders for many applications five years ago are largely solved in 2026 equipment.

The economics favor battery equipment in operations with:

• High ventilation costs (deep mines, poor airflow characteristics)
• High diesel fuel costs (remote locations with expensive fuel logistics)
• Consistent, predictable loading cycles (production loading vs. scattered development)
• Electrical infrastructure already in place underground

Diesel still makes sense where you need flexibility, long operating ranges, or rapid refueling. But the window where diesel is economically optimal is narrowing.

Bottom Line

If your underground mine hasn’t revisited fleet sizing in the past three years, you’re probably running too many small diesel units and ignoring the ventilation cost implications.

Calculate the true cost of diesel ventilation. Model alternatives (fewer, larger units; battery-electric; hybrid approaches). Include staffing realities in the analysis.

You’ll probably discover that your “optimized” fleet is actually costing you $200K-500K per year in unnecessary ventilation and maintenance costs. That’s money you could be returning to shareholders or investing in mine life extensions.

The math is straightforward once you include all the costs. Most operations just haven’t done the calculation.